Basically my car is making a humming, whirring noise still, ive ironed out most of the issues ive had with it, however this one still remains.

It had a noisy what i presumed was the gearbox when i bought it, it was only a cheap car, and the problem was basically if it was driven below 2500 revs approximately, it would sound as if you were doing 30mph in 5th gear, you know that noise it makes when the revs are too low, wrong gear for the speed your doing?

I changed the box oil and it didnt seem to really do much, though when i took the filler plug out no oil came running out which i was told it should have done, so i filled it up till it ran out and replaced the plug quick.

If you keep it above 2500 revs it doesnt make this unusual sound which can only be described as a sort of dry bearing sound, the only other time it will make this sound is when you pull away from the lights, but it literally does it for a split second, then stops.

When cruising at speed, say 70 - 80 on motorway, around 3500 revs, theres this noticeable, annoying but not particulary loud, humming, whirring, sound, it doesnt seem to get any faster with speed, i can't work out whether the gearbox is about to crap itself, if its a wheel bearing, or a driveshaft joint, any clues?

Thanks