Ok I have a 2002 Kia sportage 4x4. It is making a loud humming noise when driving. I have totally rebuilt the rear differential and replaced all the bearings in that. I replaced the U-joints and still have the noise. When I am driving I hear it, going down a hill without pressing the brake I hear it get louder, same thing with it going down the highway. I can be going 60mph and barely hear it but barely touch the gas and there it is again. Now my hubby took the rear drive shaft off, put it in 4 wheel drive, and there was no noise when driven. So we replaced the U-joints and the noise is still there. Also now there is a whoop whoop noise like a small thumping but only when i am going super slow. I have asked a few mechanics and they say to run it til it goes, BUT I can not do that. This has been going on for months now, please I am at my wits end.