Opinions welcome on this rattling clutch noise I can't figure out in my head, the audio was recorded with a remote mic sitting under the tranny so it probably sounds louder than it actually is (it's not that loud from inside the car).

rattling noise disappears when the clutch is disengaged (pedal down) but comes back on idle when the pedal is up and the clutch is engaged (around 50%?)

Makes me think its something related to either the input shaft or the clutch disk.

Possible reasons:
- input shaft bearing failure
- loose clutch springs on the clutch disk rattling around
- throughout bearing (seems unlikely since the noise occurs when there is no pressure on the bearing but disappears once the shifter fork is pulling on it)
- loose flywheel nut (read this somewhere, doesn't make sense to me)
- pilot bearing (should not be since when the clutch is engaged the input shaft should be spinning at the same speed as the flywheel)

 

Sounds like my old tranny noise but much worse. Mine sounded like a dry scraping sound with a hint of ticking added in. I narrowed it down to a worn input shaft bearing in the tranny. Do you have a automotive stethoscope? It would help quite a bit to find where the sound is loudest. Do you have any whine or growling when taking off in first/second?

 

So I just opened up the peep hole at the bottom to see where the sound might be coming from. Put a mechanics stethoscope directly onto the throwout bearing (on the ears where the fork slips in) and onto the tranny housing and the housing definitely sounds louder than on the throwout bearing. The sound also seems to be loudest at the front third of the transmission (behind the bell housing)

 

I just remembered that the noise my tranny was making was louder when I had the door open or the window down and the sound was bouncing off my fence. Was also louder when I stuck my head under the car, and checked it out on the lift.
The sound I could hear inside the car eventually started to drive me crazy.
It could be the input shaft front bearing or possibly the countershaft front bearing. The bearings could be replaced but when I looked into doing it it seems that to replace them, several other bearings would have to be replaced during re-assembly and the cost just for parts is pretty high. Somewhere around 300-400 bucks iirc. I ended up buying a low mileage trans.