I have 2000 GMC half ton coupe Seria. For several months now I have had a bumping noise that seems to originate from the gas tank area. I have taken it to two shops and each time they have said they did not hear any thing. Well that is very possible because sometimes it will go for a week without a noise and then be very noisy the next day. One idea I got that it was dryness between the springs, and we do drive on a lot of country roads that produce lots of dust.
So if one of the experts have had this complaint before I would appreciate the help.

Put it up on a lift and look around for anything unusual. It might even turn out to be a rubber mount for the exhaust has come loose.