Okay, if I've got this right.
1) Noise is dependent on road speed, not engine speed.
2) Noise is present in neutral with enough road speed.
3) Noise is present with clutch depressed and enough road speed.
If the noise is present in neutral with the clutch depressed and enough road speed, the only thing that is rotating is the output shaft. I think the rear main bearing is a good guess.
1) as long as the rear tires are rotating, the rear main bearing is rotating.
2) As long as the rear tires are rotating, the rear main bearing is rotating
3) as long as the rear tires are rotating, the rear main bearing is rotating. When clutch is depressed, the input shaft stops rotating, hense, the front bearing stops rotating.
