My car is a 1998 Camaro, 3.8 V6 automatic. The car has been running too rich for several months, but no check engine light has illuminated. Just before putting the car up for the winter, it started a strange new noise on startup. When I start the car, there is a lot of...muffled reverberation noise (?) that sounds like it's coming from the engine. It doesn't sound like an exhaust leak. What it sounds like is that the exhaust is plugged and the exhaust is getting held up in the Y-pipe area. The odd thing is that once the engine starts warming up, it goes away.

Revving it a little, I can't really tell if the exhaust is restricted or not. The car gets up to speed ok, so nothing is completely blocked. It doesn't overheat. It is idling a little rough from time to time. Exhaust pressure out the tailpipe feels normal at idle.

Is it possible for a catalytic converter to get clogged when cold, but open up when warm? I haven't seen this type of thing happen before.

The exhaust is still the factory welded system, so getting things apart to inspect internally isn't easily done...

 

None of you symptoms sound like a plugged Cat. To test for a plugged Cat is pretty simple. Just remove the forward O2 insert a Vacuum/pressure gage. Pressure should not go over 2 PSI with the engine running.