My 2003 G35C 5AT has developed really bad resonating the last 2 months.
I'm not sure if it's my factory cats (156K miles on the car) or my Invidian G200 exhaust (100K plus miles). It's giving me a head ached every other day.

Low RPMs the exhaust has two tones as if it's not flowing evenly from both pipes. Letting off the exhaust I hear a sputtering, almost like it's backfiring.

I guess it's also possible it's one of the exhaust manifolds leaking.

Any help would be greatly appreciated.

 

A sound clip would def help!

 

If you can get under the car try banging on the mufflers and resonators to see if anything is loose or sounds unusual.

How is your MPG?

 

It seems to resonate while driving. Parked reving it, I don't seem to hear it.

I'll see if I can get under the cat tonight or this weekend. My MPG is about the same, I drive 100 miles round trip for work.