1996 Suburban clanking

Hey all, on my 1996 sub i hear a clanking noise while driving. I hear it when hitting bumps mostly but sometimes when the road is smooth too, and only with the windows down. It almost sounds like a chain dropping on the ground or something. Lasts only a second. Could this possibly be a driveline thing? Its getting the tranny rebuilt on monday, i could probably have them look the driveline over while they are at it. I just dont know what it could be, whatever it is its on the outside of the vehicle. Any help is appreciated.