RW99

So, I'm breaking in a new clutch (McLeod Twin). When the engine is allowed to slow the car (say, 3000 rpm or so) I have a distinct increase in vibration and noise... it sounds like loud drivetrain or exhaust rattling. But this doesn't happen when accelerating or coasting at lower rpm... just happens when you pull a modest downshift to decelerate the car. Pushing in the clutch during this noise makes it disappear.

This is a custom single exhaust, and there's a PTK front mount turbo on the car... so the mani/exhaust piping is all over the place, and some pieces end up really close to other components. Where are the most likely places to look for this rattle, given the fact that it only happens under engine braking? Could the motor mounts be a problem?

Prior to the clutch replacement this wasn't happening. Driveshaft is a Denny's nitrous, it's fine.

Thanks for any advice...