An almost NOS Regina freewheel I got about a year ago had a clunk in it too when it freewheels......turns out the inner body was hitting some of the outer body ramps because of a pinched up area in the inner body forging that was meant to hold a circular pawl spring in place. I ground down that high area with my Dremel to clear the pawl ramps and everything was quite after that. In fact it's now my most quite ever FW. Something that might also be happening in your FW.