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5KT exhaust noise

Hi List!

I've just replaced the exhaust on my 5KT. The rear muffler was starting
to leak producing a "rumbling" sound at idle and there was an annoying
exhaust vibration between 2k and 3k rpms. I decided to let the muffler
shop fabricate the mufflers because it was twice cheaper than Walker
exhaust and prolly four times cheaper than OE one. Yes, I do know what's
the common (and particularly Phil's ;-) ) opinion about fitting cheap
exhausts, but I'm living on a limited budget and have to go on
compromises :-( So, the rumbling sound is gone, but the vibration is
still there. It happens between 2500 and 3000 rpms. It's not a "rattle"
nor a "rumble", the exhaust just gets louder and I can feel a slight
vibration through the body of the car. Once I get above 3k rpm, it's
very quiet. I can think of couple of reasons and hope you can help me
figure out the right one:
1) Both the old and the new exhausts are crappy jobs from local muffler
shops and are not suited to the engine (too little backpressure? The car
has lots of power, so the exhaust isn't too restrictive, I'm sure of
that), causing the vibration
2) Rattling cat (the air check valve at the cat was stuck open, causing
exhaust leak. An IDIOT at the dealership DROVE a bolt into the valve
using BIG HAMMER. To my surprise, the cat still works)
3) Misaligned engine (I have replaced the tranny mounts) or downpipe
(have replaced the downpipe hanger)
4) Bad engine mounts (they pass all the recommended tests, but you never
5) Cracked exhaust manifold (perish the thought. It doesn't tick, BTW)
6) Anything else to look for?

Aleksander Mierzwa
Warsaw, Poland
87 Audi 5000CS turbo (mine)
88 Renault Medallion wagon (mom's)
91 mountain bike (just in case both cars broke at the same time :-)