If a bearing is rough you'll hear it when you spin the wheel. It's hard to describe it sounds like er....a rough bearing! While you have it jacked up inspect the tyres both sides and the tread; as they spin look for obvious eccentricities.You may as well since you have the opportunity. I can't say it's usual for the noise to disappear, but then that would apply to any bearing noise unless the noise is moving out of your hearing range or it's some sort of resonance. Is it changing pitch when you change speed?
Ha well funnily enough I'm a prop designer so i've dealt with bearings before, most of which have been pretty much smooth and silent. Would a car bearing be similar in sound?
I know their internals of wheel bearings are different to standard ball based bearings. The noise does start quite high, then gets higher, as it does it really gets to your ears then it fades away but as soon as i drop speed to 60-62mph it comes back.
I'll give jacking it up as go, ill pop it on some axle stands. I imagine that not all the bearings will have gone at once so if there is a problem i should be able to tell. My worry is that the bearings will be smooth and it will be a gearbox problem....however I have got up to 70 mph, gone into neutral and let the car slow down to 62-60 and the noise still remains. Are there parts of the gearbox that still move even when I'm in neutral?
Have a good look at the tyres feel across them for any
strange or unusual ware, I had a very similar problem on a Mazda and it was the front tyres.
While I've got the car jacked up ill take a good all around look of the tyres! They were all new though at my last MOT in august.