if it happens also when you're braking. I'd start looking at bushings. Radius arm bushings, axle pivot bushings. You really need to try and reproduce the sound when it's sitting still, if that makes sense.
Burt- 1989 Ford F250 7.3 NA, Zf, 3.5" Straight Pipe Exhaust, 2001 SuperDuty Axles With 3.73 Gears, 5" Lift Using All Factory Parts, Captain's Chairs, G31 Batteries, NorthStar NS3000 CB With RK56 Mic & 102" SS Whip, Aircraft Landing Lights & 102dB Backup Beeper.