This has been an on going problem since replacing my exhaust pipe and muffler. At first some family members as well as a mechanic told me it was the transmission making this noise. I took it to a transmission shop and he told me nothing was wrong with the trans and that it shifted well. He put the car on the lift and started taping around and when he hit the cat the noise was similar to the noise I was hearing. BTW he drove a 87-91 Bonneville too and he loved his car so he definitely knew about the Bonneville trans.

So fast forward, I changed my oil and after pulling the car off the ramps, the noise stopped as if the piece of *to be determine* had fallen back and this has lasted until last Friday. The noise is back and the sound has changed. It only makes the noise when I'm in Drive and when I'm holding the brake pedal. You will notice in the video when I put the car in park the noise stops. Also when I'm driving I don't here the noise. It also robbing my horsepower a little. I believe it may be the cat.

Sounds like a heat shield , do you have one? If your losing power it probably is the cat.

I don't know, where can that be found?

There usually is one over the cat, may have fallen off already though.Sometimes its loose and rattles.

The heat shield on my truck rattled. I drilled through two of the rivet holes and bolted it down. That kept the rattle away for a couple years.

Update:


So I went out to check to see if the heat shield was the problem and while taping on the cat I heard the noise loud and clear. It is cat, sounds like something inside of it has broken off inside and I can see how air blowing out the engine can cause whatever is inside of it to rattle around. About how much do you think it will cost to replace the cat at a shop as I don't have the tools to do this myself?





Here'* more update video

Yep cats done, I am interested in price also.Good luck.

The continued rattle after a single striking blow is not typical of cat-media inside the body. It definitely sounds like the heat shield. To be sure, you should be able to get a finger nail or screwdriver blade between the heat shield and the side flange (joining the top and bottom halves) of the converter. If there is a gap; that is what is causing your rattle.

yup, its your cat, while i agree that the heat sheild is probably loose also. take your hand and wiggle the entire exhaust system (while cool) and you will hear all the pieces falling apart in there.

expect to pay $50 for a new one on ebay, and probably $50 to have it installed

*assuming your exhaust is 2.5"
http://cgi.ebay.com/ebaymotors/CATAL...QQcmdZViewItem