I just finished a complete exhaust system, hedders|cat|cat-back and the car idles when warm in drive at about 500-550RPM and vibrates like a SOB. If I kick into neutral it's fine. How can I adjust this?

Thanks.

 

I have a similar problem, after installing my headers, i noticed that i get more interior rattling and squeaking noises. And the car makes this strange pulsating vibrating noise when warming up in drive.

 

It's called resonance, and it sucks. I had the resonator rust off of my subaru, so I just welded a strait pipe where it once was. Worked fine and was no louder than it was before, but now the car gets a horrible vibration at idle in gear. Subaru had it for a reason I guess.

If it really bugs you, you might try installing a short "glass pack" style, or a single chamber flomaster style muffler somewhere in the system, I've had a hard time trying to source an actual resonator. I've even noticed that the diameter and shape of the exhaust tip used effects resonance.