I'm not sure what post i said that .. i've posted a few times
A bearing .. let's take a wheel bearing for example .. the fault is basically a tiny knick out of the metal roller surface or possibly a ball bearing .. the surface is the norm though .. think of a small hole ..
Trust me they can be tiny and you don't get play or anything obvious to help locate this fault .
A light pressure on a ball bearing as it goes over a hole and not much happens .. put pressure on the bearing and it digs into said hole a bit more ..
its causes a noise and it will be dependant on load .. ie it will whine more on a left hand bend than on a right hand bend due to load shifting on the " hole " .
Take this as a ballpark explanation though .. im sure there are more factors at work but either way its load dependant .
A diff bearing noise .. well to me .. the load is there on acceleration and deceleration ( the bearings are at 90 degrees from a wheel bearing ) .. hence you can get a whine at these points .. get it in the sweet spot of no load .. cruising .. and the noise stops .
More common on the earlier air cooled cars .. at least that's where i see it more due to age i guess .
A front diff will also cause a noise but it's much quieter than an early gearbox shall we say .
It's never a problem as such .. it's a noise and these are annoying hence people have them repaired .
I've not seen a collapsed wheel bearing since the taper type of years ago .
Anyways .. IMHO and my way of thinking / explaining things .
