Cats often make a rattling-type sound when they go bad. The baffles or whatever is inside of those things starts to break loose and rattles around inside. However, you can usually hear the noise at idle not just at certain speeds...

IMO - I would thoroughly check your exhaust system from front to back for loose or damaged pieces. As the previous poster noted, I would concentrate on the heat shields first.

A couple of other things to check:

1. Tightness of exhaust manifold bolts and especially the manifold to downpipe bolts, cat bolts too while you're at it. Look for slipping or damaged gaskets while you're there!

2. The heated air hose that runs from the exhaust manifold to your airbox. The one that supplies warm air to the engine when the car is still cold. This hose, like heat shields, can loosen, crack or even break off.


Good luck!


Jim
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90 244DL 263k :)