Toooony I'm tellin ya dogg it's dem BJ's... It's about that time mayn
On a serious note... Check the bushing's on them. If they're clunking I'm gonna take a shot in the dark here and say that your bushings are shifting when the bars are flexing. Which will most definitely cause that clunking. Also another shot in the dark here but did ya use the right size bolts? If they use different bolts than stock, say larger bolts, and your using stock sized bolts, which were saying are smaller, the difference in bolt size would definitely cause the bars to shift.
When ya get a chance jack that bad boy up and snap some pics of the mounts on the rear and mounts/bushings on the front! That'll help us, to help you