99 1500 Noise coming from rear end

[gregbz]

I have a 1999 Sierra 1500 that once in a while has a very load noise coming from the rear of the pickup. The pitch of the noise does not change if I speed up or slow down but once you come to almost a stop the noise stops. It alway takes a bump to set the noise off and it happens only after the rear wheels hit the bump. Have had it to a mechanic who thought it was the transfer case. He has opened the transfer case and also the rearend but can't find anything wrong. Could something be throwing the drive shaft out of balance when I hit a bump? Any help would be greatly appreciated.