I have a 2001 blazer 4x4 that has had a consistent rattle/thumping for over 5 years. The noise happens when I am traveling between 35 and 45 mph, and neither accelerating nor braking...just chugging along mostly coasting. It stops if I accelerate or brake. It is not affected by road condition. I most often think it is coming from the back end, but am not 100% sure. The noise is not rhythmic like I would expect if it was tire related. Any ideas?

 

possibly the torsion bar mounts are broke. Not a huge deal,it keeps the torsion bars from clunking etc from suspension travel. If you follow your torsion bars to the rear,where they run into the crossmember there is a mount on each side of the trucks frame,that holds the crossmember down. you can push up on each end with a crowbar and if the crossmember moves up any,they are broken. You'll have like a crossmember sitting in the frames crossmember. make sure you push up on the member the torsion bars are in,and not the trucks frame support.