I posted few days ago about my cat. converter making some noises and it turned out that the ceramic part in it was broken and it was making the noises. Anyway, i purchased a new aftermarket cat (not Audi brand) and i installed it today. It firt perfectly.
However now after i test drove the car for 20 miles i see a little difference in the car behavior. The RPM didn't change at all and they are stable as before at around 750-800 on idle.
The difference is that at around 2100-2200 RPM only i feel some kind of a slight vibration from somewhere. Below and above these RPM there is no vibration and car is running OK - no vibration.
What can that be? Help me with any ideas what i might have done wrong? Please let me know what you think.
Also, now the sound from the exhaust seems louder somehow. Is this normal? Car Performance is OK but i have the feeling that it had decreased a tiny bit. (that's just my first impression -haven't gone faster than 65 mph yet)
Did i make a mistake when i put this aftermarket "cheap" cat converter? The dealer asked $1050 for new Audi Cat!!! my car is Audi 100 I5, non turbo.

Thanks for your help!

Had a similar problem on my car .turned out to be the lefthandside gearbox mounting