Idle Noises-SOLVED

This evening, I asked my dad to help me diagnose the rattle in my car's engine bay. After a brief trip down the road and a listen to the whirs and thunks emerging from the firewall, he suggested that the problem might be with the throw out bearing in the transmission. Looking into this possibility, I discovered that Mazda and Ford manual transmissions commonly produce something called "gear rollover noise."  This sound is generated from the teeth of the transmission gears as they collide, while the car is not in gear.  This noise is the result of vibration from  the engine carrying through to the transmission. In some transmissions, including that of the Mazdaspeed3, the actual gears are designed primarily to engage quickly and move efficiently, as opposed to moving with buttery smoothness and operating silently. This choice of goals results in additional noise when the gears are not engaged, such as during the engine cool down period when the transmission is in neutral. Problem solved. Of course, if the gear noise ever continues when the clutch is depressed and a gear is selected, then the throw out bearing is probably shot.  As a side note, if you are having a similar issue and would like to reduce this noise, you could a) try very heavy transmission oil, b) install a much heavier, dual-mass flywheel, or c) purchase a custom-made transmission that has closer tolerances between the individual gears.