thumping

I have a 1996 chevy blazer 4.3 4x4. I noticed yesterday that while driving i have a thumping noise coming from what sounds like right under my feet or a little to the center. The faster I go the faster it is and of course the slower I go the slower it is to the point it stops when I stop.

Does anyone have any questions or suggestions?

constant only over bumps,trans mount, u joint, mount on front lateral bar?

I figured it out. Apparently when I changed my brakes I didn't tighten some of the lug nuts. I had two that almost fell off on the front driver's side and the same with the rear driver's side. As soon as I thightened them up it was back to normal, no noise.