: Mysterious metallc rattle that no one can figure out

07-11-11, 06:30 PM
I have tried looking thru the archives to find an answer but haven't been able to zero in on this issue. 1976 FWB 500 cu in carburetor engine, upon light acceleration there is a metallic rattle or clatter noise it sounds like its coming from drivers side area from under the hood. Its only on light acceleration though, if you put your foot down harder it diminishes, then harder again and it goes away completely. It only does it once the engine is well and truly warmed up for a while, but is not so bad if the outside air temperature is cold. Hot weather makes it worse. It only occurs if car is moving, if you put car in park or neutral and lightly apply gas pedal it doesn't do it.

I used to run it on 91 octane gas but the last few tanks I have used BP ultimate 98 octane which apparently has wondeful cleaning agents to clean sludge and stuff from the engine. I have now been putting upper cylinder lubricant in as well. Timing was adjusted, made no difference. Tried disconnecting EGR valve, no difference. I thought it might have been the damper door in thermac air cleaner but that is working as designed. There don't appear to be any loose objects rattling around in the engine bay. Throttle linkage also appears ok too. I also replaced a leaking rocker arm cover gasket on driver side a few months ago, all the pushrods and rocker arms looked fine. (the FSM suggested problems in valve lifter/rocker arm/valve train could cause rattling). I'm running it on 20w 50 oil.

One possibility I came up with is carbon deposits in the cylinders getting hot enough that they start igniting the fuel? The engine starts and runs beautifully its just this rattle which has been going for many miles now I'm concerend that it could cause further problems down the line, everyone I have asked about it is stumped

07-11-11, 08:56 PM
Couple of possibilities:
- exhaust manifold heat valve
- catalytic converter heat shield

07-11-11, 10:11 PM
I agree with your last paragraph; detonation from carbon deposits caused by too much slow cruising with high octane fuel. There are a few ways to clean it out and I have experience with none of them. I think GM makes some stuff to dump in there, there is the water technique and I think Seafoam is supposed to remove carbon. Somebody may come along soon and explain them better than i can.

07-11-11, 10:22 PM
I would say it's the exhaust system being that you can't replicate the sound in park or neutral; the noise isn't internal to the engine, but vibration related.

But if you want to clean the engine, I recommend Sea Foam. You basically pour half the can down the carburettor, while some one holds the idle up, and them drive around from a half hour. It cleans the entire induction system and removes deposits from the top of the cylinders.

07-11-11, 10:56 PM
I know this will sound crazy because it sounded crazy to me until I was able to see it. I chased a rattle in an engine once for days. It sounded like a cracked flywheel. After I checked the flywheel and realized it was good I pulled the transmission thinking maybe the front pump had the rattle. Once the trans was out I noticed the starter drive wasnt fully retracted. Turns out the spring on the starter drive had broke and wasnt retracting the gear. The rattle I was hearing was only under light acceleration and would eventually diminish the faster the car went. The acceleration of the car was enough force to slide the starter drive out enough to contact the flywheel. After getting the starter out I suspect the rattle would occur during hard accelerations also but I wasnt able to hear it. Its probably a longshot and Im the only one this has ever happened to, but its worth mentioning. Good luck...