clunk while cornering

For the last 30,000km of the 100,000 i have on my 2000 sierra I have heard a heavy clunk like some thing shifting under my feet running back to the box . this clunk does go back and forth at first it was intermittant but now it happens on every 90 degree corner. GM cannot find it and my mechanic whom I have great confidence has not found the cause with a cursory investigation. I hope i'm able to find the solution