Originally Posted by Veloce300DT

If you havent checked the air filter bracket, it might be that. With bad engine mounts, the stress put on the bracket from the shaking and vibrating makes it break, letting the air filter housing rattle free atop the engine. The vibrations smooth out at higher rpms, and the housing stops shaking, then when u return to near idle and at idle, the housing starts rattling again.

Gotta fix mine asap, of course after the engine mounts are replaced!