I have 2000 suburban 1500 keep hearing a thumping noise at 5 to 15 mph while backing up or driving after it goes away at fast speeds.need help here.

 

being that it happens at low speeds, would be pretty easy to have someone drive it as you watch it go bye. A tool called 'chassis ears' would be ideal in this situation.

Not affected by engine speed?
Under load?
Only reverse?

 

it only happends at 5 to 15 mph constantly.in reverse and drive it make the noise.

 

Have you inspected the underside of the vehicle for:
Anything wrapped around the drive shaft
Checked the U joints front and back
Jacked the vehicle up off the ground and rotated each wheel in both directions checking for a wobble or anything stuck in the tire
Inspect the transmission and motor mounts for cracks or tears...

Think like a thump.. those are the things and places I would check...

 

My 02 Suburban sometimes makes a weird thumping noise. It's almost always when I'm just backing out of the driveway, but once or twice has done it when I'm less than a block away. Doesn't always do it. Only does it once, when it does it. But it sounds like something is dropping near my glovebox. In fact, I always look over there because I think it's actually something dropping. lol. Does this sound like what yours does?