Dave, correct me if I'm wrong, but the chattering noises related to a throwout bearing only occur when the clutch pedal is NOT pressed. Usually the easiest way to determine if it is the throwout bearing is when the car is idling in neutral it will make the rattling/chatter noise, but as soon as the pedal is pressed it will go away.

Maybe I misread the posts above, but it sounds like when he has the pedal pressed in while in neutral or during shifting, the noise is the loudest...

1999 Eclipse GS
Built motor
Hahn 16G Turbo w/ FMIC
Hahn Stage 5 Portfueler (270's & 500's)
Zeitronix Zt-2 wideband w/ 3.5bar MAP
3" SS Turbo Back
CM Stage 4 Clutch and Fidanza Flywheel
Baer Big Brake Kit