I have a 1999 Silverado Z71 with 134,000 miles on it. Purchased it last month. The front end growls at lower speeds then increases in pitch as speed increases which is quite annoying. Also a slight vibration can be felt while the front end is humming at lower speeds. In trying to determine the problem I assumed it was the bearing on the driver side. I purchased some bearings and went to remove the wheel in preparation. While the wheel was off, I thought I'd take and turn the rotor/hub and could hear/feel something like what would produce the sound. However, it wasn't really coming from the bearing when I put my ear near it. I listened near the differential and turned the rotor while putting my hand near where the short axle enters the differential. I could feel a slight vibration and hear the sound and realized that the differential may be the problem. I checked the other side, long axle side on the passenger side and it is smooth/quiet. I did notice that there was a mess of oily dirt on the skid plate under the differential and on the differential itself. Leaking unit. I removed the filler plug and then proceeded to add 18oz of gear oil. Ugggh. So it was low and the bearing or internal components are bad? Well, I found someone on ebay selling a 99 Silverado/Sierra front differential that would work and the guy told me it had less than 10,000 miles on it. When it showed up, I thought I'd spin the axles and have a smooth spin. Nope. Just the same sound/feel as the one in my truck!! The long axle shaft side is quiet/smooth like on my truck too! Now I'm wondering if this is normal? There are rubber bushings on the mounts of the differential that would insulate vibration so the little vibration/sound wouldn't be what I hear/feel if it's normal, right? Or is it the bearing? Maybe the bearing has to be under load/on the road for it to make noise? Maybe the guy who sold me the differential said it had less than 10,000 miles of USE...and not less than 10,000 on the truck it came off of? Someone here must know something!