Try oiling the freewheel. It may not solve anything but it never hurts.

Pull the wheel off the bike and lay it down on its non-drive side. Get a can of light oil (best one with a spout, 3-in-1 works just fine). Spin the freewheel backwards and look for the small gap between the spinning freewheel and the fixed freewheel body. Dribble oil onto that gap. All around. Use lots. Keep oiling until you see oil coming out from the bottom of the freewheel (between the freewheel and the hub). This should quiet and smooth your freewheel and might solve your clunk problem. If not, well you have just done a good maintenance practice which is often neglected.

Ben