Could it be a driveshaft? that would make a louder noise as you reached higher revs and make no noise on idle
Or does it not do it all the time?
Just thinking a bit more on it, I would expect driveshaft/cv joint faults to be vehicle speed related rather than engine speed.
Also have you tested whether it makes any difference pressing the clutch while the noise is happening? obviously the revs will drop off but does that then make your noise drop off.
not noticed mate, when i decellerate, the noise dies off, increasing when accelerating, no noise on standstill in neutral revving.
if i depressed the clutch in motion the noise would go away yes as i would release the accelerator wouldn i, so its entirely rev dependant