[st8w8]

I have a 94 sts and something is wrong with exhaust. Last week I had the both mufflers replaced and it quieted down alot. After leaving muffler shop I kept noticed a rattling noise when accelerating. My guess was the cat. It goes away and comes back. With the car park and idle the car is pretty quiet but when I accelerate I either get the rattling noise or rumble. The rumble feels like its right under my feet when I'm driving. Also on the highway it does this thing where it vibrates the whole car for maybe 5 seconds and goes away for maybe ten seconds and comes right back and repeats itself the whole time I'm on the highway. The sound and feeling of is like when you get too close to the dividers on the highway and and you run over those speed bumps or like the things that slow your car down when you have to stop at a toll both. No codes except for P036 but I'm going to by a brand new EGR valve I've cleaned so many times and replaced with used ones and the code comes and goes .

What I want to know is where to start? I have some extra money right now and I'm willing to pay however much to solve these problems.

 

[Submariner409]

Check and align the muffler/exhaust system hanger cookies. On some of these cars - 1/4" off-center and a pipe hums on the frame under either acceleration or deceleration.

 

[Ranger]

Start by taking it back to the shop that did the exhaust work.

 

[N*Caddy]

I think I know what it is:
Check underneath your car behind the CAT there is a body cross member (goes underneath the exhaust).
When you accelerate the exhaust touches this cross member. If you have less than 2/3" clearance that's the issue.
You have to check if the exhaust rubber hangers (right behind the cat) are not sagging. I think I have a picture somwhere...
Here found it:

On the right side is the back of the cat (>>front of the car) and on the left side is the front of the silencer.
In the right side of the picture you can see one cross member (witch I believe '94 does not have it) and symmetrical to the left (not bolted in the picture and oriented slightly diagonally over the silencer) is the other cross-member. This is the one '94 has it too and is the one causing the noise.

 

[st8w8]

I will let them know all of this. I'm supposed to take the car to the muffler shop in the morning so they can figure things out. They do pretty good work but they're the type of people that don't fix things if you don't ask.