I've got a humming coming from my car and I can't determine where its comign from, its a fairly loud hum but not a whine like normally a diff would whine imo
its loudest at around 30mph under load. it almost sounds like a front wheel bearing but when i feel the hubs after driving for hours it is still cold to the touch.

what can i check on my diff that might make a humming sound rather than a whine?
I have 3or 4 year old Ujoints on my driveshaft, do they wear out that fast? any other idea that might cause a driveline hum?

 

I heard a lot of the early firebirds do that. I figure it has to do with the design of the rear end. Mine does it too, I took it in when pontiac gave a recall early on (though they didn't say what caused it) and it still does it. Car drives great, so I haven't worried. By the way, mine's an 84 and no problems yet. Only info I've read is that it may be a poorly designed oil pressure regulator, but that doesn't make sense to me.

By the way, is your car posi or not? Mine's non-posi.