Off Throttle Grinding

[deftoneZ]

Hey everyone, I'm have a weird grinding noise when off throttle whenever I'm above say, 3k RPM or higher as the engine is slowing down. It eventually stops when a bit above 2k RPM. Its very noticeable when in manual mode (its an auto)

It doesn't sound like the VQ "hiss" people talk about. The best I can describe it, is similar to maybe a stone on a grinding wheel, kind of a scoring type noise.

I have Z1 high flow cats, HKS Hi Power Exhaust and a tune by Z1. The car started doing this the day the exhaust was put on. I've not been able to identify an exhaust leak and I've checked the pads and rotors for a lodged stone, and nothing.

It doesn't seem to come from the front of the car, its more towards the rear I'd say, at least behind the seats and towards the hatch. If you have the windows down and are driving by some kind of barrier off throttle, its very noticeable and kinda sounds like crap.

Oddly, my 350Z (a DE motor) had a knock off HKS exhaust and test pipes and I remember it doing the same exact thing.

Any ideas on what this may be or how to stop it? I can't seem to find anything on this sound, maybe its an auto only thing?