Thanks for your suggestions, Jim! Ok, here's exactly what happens: Sitting in neutral, if you rev the engine any amount, there is a rattle noise. It only happens as the rpms increase, though. If you hold your foot steady on the accelerator, it stops (i.e. if you hold steady at xxxxrpm). Sounds as though it's coming from the underside toward the front - I think this is the area of the catalytic converter...? Can't see anything vibrating or moving when you look under the car (not on a jack). Oh, clutch engaged or disengaged makes no difference. Any ideas??