How to fix a clanking rear wheel/flywheel
From the comments above, it seems that it really is a free wheel that is making the noise.

Apparently cheap free wheels can be like that. Lubing them can help. This discussion has some extra info: http://forums.roadbikereview.com/fixed-single-speed/fix-shimano-freewheel-clunking-noises-150432.html

TL;DR version: Get some gear oil or grease gun and apply liberally.