Well.. I had a 280ZX, and now own a 200SX... and both cars made the same "clunk" sound. It bothers me too! I asked the Nissan dealer about it... the guy said "possibly", the rear frame/unibody is twisting or bending slightly, or the mount is shot in the rear end. This would cause the it to sway around a bit, and hit the floor. Personaly, I think it has something to do with the rear suspension... but I cant find anything wrong with my 200sx. (or the 280zx I had) I have yet to loose a part of my car, and it still drives well. I think all you could do is replace EVRYTHING in the rear end of the car... suspension, u-joints, bushings, ect. but that would be expensive and may not fix the problem anyway... I think it's just a little... quirk with Nissans. Dont worry man, you are not alone!
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The crazy datsun guy