its probably the input shaft bearing and/or the bearing on top of the main shaft
I dont know why people tell others that its the damn pilot bearing...if you press the clutch in and the sound goes away, pressure is on the pilot so it would make the sound then, and not when the clutch is not pressed.

I really really suggest you change the input shaft bearing, counter shaft bearing, as well as the bearing on top of the main shaft, if youre going to do this at all...maybe even the bearings on the diff. Sure would suck to change one and it not be the one that was bad in the first place...