rear end noise

I have a 2004 Tahoe LT 4x4 with 4000 miles. It has always had rear end noise when coasting at around 45 mph. It usually goes away below 40 or above 50 except for a little noise at 30mph. The dealer says that it's normal and of course my wife doesn't hear it. Anyone else have this noise? If so was anything done about it?

Ron