I run 375lb in the front and 550lb in the back in my Fiesta (1650Lbs).
I use to bottom out in Turn 17 at Sebring...the lower control arms would hit the frame but I'd never really hear it happen. BUT, Sebring has extreme bumps (even jumps).
I agree with another post that something is loose or broken.... maybe a ball joint, strut mount or even a steering rack.

On my road car (BMW 5) the roll bar links have some play in them and they would clunk sometimes.