As with any diagnostic task one must first understand how parts interact as a system, and then apply logic and isolation to determine the source of a problem. Otherwise you will just thrash around in trial and error mode, which not only takes longer but which may make things worse, or at least even more complicated to figure out.
If you can hear and feel something pronounced enough to be called a "clank," and there is no sign of it during pedaling, then it simply cannot be anything but the freewheel. However, as the freewheel "mechanism" consists of 2 pawls that each operate multiple times per wheel revolution, and ball bearings that operate continuously, it would not be the mechanism. The only thing in the rear moving in relation to another part once per wheel revolution, and that is not doing so when you pedal, is the outer part of the freewheel in relation to the body that is attached to the wheel. Whether pedaling or coasting the cups, ball bearings and cones are moving in relation to each other in exactly the same manner. If anything there are some hub related problems that would show up when pedaling and not when coasting.
I would conclude that the noise is caused by some defect (crack or similar) in either the outer or inner freewheel body.
As always I may have overlooked something - I'm always willing to acknowledge oversights and acquire new information. I'm going into detail to show the process rather than to exhibit my knowledge.
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Last edited by cny-bikeman; 05-27-12 at 04:55 PM.