: '95 FWB Driveline "hum"?



caddycruiser
04-03-06, 01:34 PM
Noticed this before, but didn't remember about it until my father mentioned it on the phone yesterday after driving my car.

Though pretty much silent most of the time when cruising/lightly accelerating to speed, RIGHT from 46mph and through 53mph EXACTLY, there is a quiet but high pitched sort of howl/hum. I only noticed it on the drive home first because the radio and fan were off (a little of either makes it unnoticeable), but thought it was odd that the hum is present only in that small range of speed and never any other time.

It sounds more like it's coming from the rear. And I just had the trans fully serviced and inspected, which checked out A-ok, so I'm pretty much certain it's the rear end.

Any ideas on what, exactly, might be causing this 46-53mph hum/howl?

Like I said, the car sounds and runs flawlessly otherwise, but this little noise is making me think something is either worn out or needs serviced.

Katshot
04-03-06, 02:51 PM
Any vibration associated with the sound? Does the sound come and go if you modulate the accelerator (coasting, vs. light accel.)?

caddycruiser
04-03-06, 03:14 PM
It's smooth as silk and vibration-free the entire time, and the noise is ONLY noticed when accelerating up to speed from 46 to 53 mph. Don't think it matters how hard or light you are on the accelerator, as I've noticed it under a few different loads.

Right at 46, it's kind of like the hum is turned on then immediately flicked off at 53 mph, which is what seemed odd to me.

I say "from the rear" because in the driver's seat, that's the best guess of where it seems to be coming from, but we could be mistaken. Especially since it's only present in that small range of speed.

Katshot
04-03-06, 04:30 PM
Once you're in the meat of the noise zone, if you release the gas pedal and glide (or at least let up a little), what happens to the noise?

BCs71
04-03-06, 04:59 PM
Hmmm...

Possibly the Torque converter locking/unlocking? That is pretty close to Overdrive speed as well, so you may be in the "gray area" of the 4-3 downshift or 3-4 upshift.
I noticed this is kinda sloppy in both my 96 Caprice and 94 FWB.

caddycruiser
04-03-06, 05:17 PM
Once you're in the meat of the noise zone, if you release the gas pedal and glide (or at least let up a little), what happens to the noise?

Well, I can't remember precisely, but I'm pretty sure it stops or at least lessens if I let up.

And actually, now that I think of it more, I think this is all right before it shifts, so maybe that has something leading up to it...it's right before the 3-4 shift I think.

It's not exactly a "whoa--what the heck is that?!" kind of sound, just more of a strange high-pitched hum. Actually, there's the same kind of noise, but a LOT louder, in our '04 Suburban when you let off the gas or just ease up on it, around 40-55.

caddycruiser
04-03-06, 10:00 PM
Should I be worried at all? It does do it right as it builds up to do the 3-4 shift, and then stops, so I still wonder what it might be...or why it does sound like it's from the back, when it might not be.

ocjmakaveli
04-04-06, 10:11 AM
Well have you had the differential oil level checked? possibly a bit low ...

There's many many possibilities I think it'll be hard to diagnose even in person let alone over the net.

The one other thing is also that noises are only apparent at certain speeds although they might be occuring the whole time.

What I've noticed is sometimes if you roll down the window you can hear odd creaks or so that otherwise you wouldn't hear OR if you roll up the windows and with everything silent you'll hear little odd things(some of which I think our minds create out of boredom).

Best fix is turn on the radio or leave a window slightly open because when you think about 200 lb. pieces of metal turning at 1000's of rpms etc. something is bound to make a noise somewhere along the line.....

Chasing this small problem will most likely cause misdiagnosing and much much wasted $.

One thing I would do personally if it bugged me enough would be lift the rear on jacks and get it upto speed and check if I can hear it on the outside if not then I'd know it's most likely from within the trans or diff. and the sound is being carried throught the car to the interior.

caddycruiser
04-04-06, 10:41 AM
Good points. It really doesn't bother me, except for just wondering what it is.

And given that it's always from that one speed to the other, right before it shifts, I wondered if anyone might have a suggestion.

Having the diff fluid checked is probably a good idea though, and one I was about to do, even though it seems solely related to speed. I've tried listening for it other times, but it's only at that point while accelerating.

The idea about the torque converter is sticking with me though...because it's right as it "spools up" to the 3-4 shift, then disappears.

ocjmakaveli
04-04-06, 01:34 PM
The one other way would be to get one of those data tech things and hook it up then you will know exactly if it's the torque converter.

Are you absolutely sure it doesn't make the noise at any other speed?

I also think it might have something to do with rpm and possibly the hum may be just some unusual engine noise traveling to the back of the car.

I've had so many occasions when a noise in the rear was caused by the front and vice versa.

Good luck if you ever find out what it is be sure to post back for future reference

N0DIH
04-04-06, 05:12 PM
Try it in different gears. That rules out trans/engine. 2nd will do that speed easily, 3rd too.

Driveshaft won't change under load/not under load. Driveshaft noise is there all the time (vibration). U-Joints tend to squeak or clunk when load is changed.

Rear end gears (not rear end noise in general) will change if loaded/unloaded as Katshot is digging info for. With a limo fleet, he likely knows this proceedure well.

Does it get worse with heavier throttle or is it only a cruise/light accel noise?

Under slight acceleration present or not?

Does it do it with the brake pedal VERY slightly pressed (enough to light brake lights, but not to apply brakes) to rule out converter induced noise.

caddycruiser
04-04-06, 06:53 PM
The only time it makes the sound is right from 46-53 mph under acceleration, light to heavy--doesn't seem to matter--which is right before it shifts, I think.

I've never noticed it any other time, and have been listening for anything odd, particularly not when just sitting with the brake pressed, or at least I don't think so.

It's just always as it winds up to speed, but only in that small range--not below 46 and not above 53 (or at least not to the level that either of us have heard it).

N0DIH
04-05-06, 01:39 AM
Check rear end oil....

Is the gasket the OEM Factory one or a replacement one?

caddycruiser
04-05-06, 06:31 AM
Check rear end oil....

Is the gasket the OEM Factory one or a replacement one?

Never checked it yet, so both are on the list right now. I'll just have to do some more experimenting and checking to figure it out.

I have to find the last owner's phone number too, and chat with them--I had it, but can't find it at the moment. Though the dealer didn't know exactly, I'm pretty sure they traded it on a new 300...since it came as a new car trade from a Chrysler dealer lot.

Regardless, I just have to try and find out some actual service history...and keep digging around to inspect.

Katshot
04-05-06, 07:11 AM
Well, I can't remember precisely, but I'm pretty sure it stops or at least lessens if I let up.

And actually, now that I think of it more, I think this is all right before it shifts, so maybe that has something leading up to it...it's right before the 3-4 shift I think.

It's not exactly a "whoa--what the heck is that?!" kind of sound, just more of a strange high-pitched hum. Actually, there's the same kind of noise, but a LOT louder, in our '04 Suburban when you let off the gas or just ease up on it, around 40-55.

Not that I'm SURE without actually experiencing it myself but your comment about the Suburban having the same noise reinforces my original thoughts of it being a pinion noise from the differential. The noise is common in the trucks because they use straight cut gears and tend to be a little noisier than the cars. The heavy-duty limo chassis used to use the straight cut gears from the trucks but were eventually replaced due to customer complaints about noise. If you're getting this noise in a regular sedan, there's a problem. Could even be axle bearings too. These are all common issues on these cars unfortunately. I'd start by opening the rear and checking gear wear patterns on the ring/pinion gears, and then slide the axles out a little to check for wear on the bearing areas.

N0DIH
04-05-06, 10:28 AM
I went round and round on differential noise on my 96 Burb and ring gear and pinion noise (IIRC) were always on noises. Ring and Pinion noise was under load only. Axle bearings will change under side loads, aka, take left turns vs right turns the bearing will unload and quiet down or get noisier.

Differentials aren't noise free, they will make some noise. My 96 Burb rear end (3.73's in the 8.5, and now 9.5) are nearly silent, my Cadillac is slightly noisier, but it might be because the rear end is farther away (wheelbase in the burb is longer....) Hard to say. So some noise is considered normal. A good differental shop can easily tell you if it is normal or not.

Ensure that the diff fluid is ok, mine drained out and I lunched the rear on my Burb due to the pinion seal leaking, and with the cold, the oil stuck to the underside of the truck, so I never saw it. Cost me $400 to replace it, and I lost my posi too. But gained a 14 bolt semi floating 9.5" rear end. That was well worth it. This is the same differential installed in the Trailblazer SS now, so it is proven to 400 hp.

caddycruiser
04-05-06, 11:13 AM
The Suburban is 2wd and it actually has a howling noise to it, only when you let off or ease up on the gas from cruising speed. Over on the Escalade board, I was going through a thread where a lot of people had the same kind of noise, but it ended up being their front differential that was the problem and needed replaced--all AWD models, though. Could still be the rear end, though, but have to get it checked out. Sometimes it is just REALLY loud, which has always bugged me, but there hasn't been any "mechanical" issues in the 32k miles.

The noise in my Fleetwood is a lot quieter and less noticeable, and only seems to be at that one spot in acceleration--not when letting off the gas--and kind of like something is spooling up, right before it shifts. Or at least we think so, since this seems to be the only time it's been noticed so far.

Actually, you know what the Fleetwood's noise is a lot like? Kinda like the sound of certain tires on concrete or grooved highways, except for the fact that it comes and then goes away in that speed range.

Need to have the rear checked out, though, and probably in both vehicles.

N0DIH
04-05-06, 12:49 PM
Honestly I think mine does the same, around the same speeds too, but I am 99% positive it is just differential noise. But as to what is good/bad? Don't know. When I can't hear my wife talk to me in the truck (when that one went bad) and she sits next to me, that is bad. Eventually it got to a point where it took a lot more power to go too. I deadlined the truck after that.

When I took at a look at the ring and pinion, there was a significant amount of wear on the toe side of the ring gear. The heel was ok, pattered with the marking compound just fine. But the toe side was waaayyy off to one side, not centered. So if I changed the shims, it would screw up the other side. I think there was just too much worn away to fix. It was cheaper to put in a new rear than fix that one. But that was LOUD, and it came on suddenly after a 40 mile highway drive. But the wear was happening over a long time. No additives fixed it. That Lucas oil helped for about 30 miles, then got really bad.

My burb was slightly better with 4WD on, then off, but that wasn't a fix.... That is how I pinpointed the rear end.

caddycruiser
04-05-06, 01:03 PM
Honestly, the noise in the Fleetwood is so quiet, I'm not overly worried about it, more curious as to what it might be. Still going to check the rear end out, though, just to make sure.

Now the Suburban, I've thought that needed checked from day one, but the main driver (mom) said she doesn't notice it and isn't going to worry about it yet. Dad did though, and mentioned "What is that?" before.

But thanks for all of the tips, I'll surely take them into consideration and appreciate it.

Katshot
04-05-06, 02:21 PM
The carrier shims aren't usually the problem. I've usually found the pinion depth to be the culprit. Usually, I find that somebody replaced the pinion seal and then retorqued the nut on the yolk improperly. The crush collar only works in one direction. Once it's over-torqued, backing it off doesn't fix it. You have to replace it. I haven't done many of these so I'm no expert but the few I've done have usually been the problem. Also, pinion bearings can make noises on and off varying with load too. IMO, setting up a rear is something only a pro should do. There's a lot of room for errors that only experience and technique will help you avoid. Hell, I had a buddy of mine do my rear, because I wanted it RIGHT.

tiaracherrelle
04-25-06, 10:54 PM
I have a 93 and mine makes that same annoying humming noise, but I've got a new transmission and torque converter. It lets up when I take my foot off the gas. No one can tell me what the heck it is though...I've had the car for a little over a year, and I haven't had any problems yet, so may be it isn't anything too serious.

The Ape Man
04-26-06, 10:01 AM
You might check with the dealer for transmission additive. IIRC there was some happy fix it in a can for lockup torque converter noises. Maybe your trans fluid had this stuff and it was not replaced during the service.

The Ape Man
04-26-06, 10:04 AM
BTW, check with the dealer and not a general auto parts store. Someone here might be able to find a GM happy bull-iten on this too.