Cheap freewheels seem to all have this issue. I have a Dicta and it does the same thing. Sometimes it'll be silent for 20 miles, and then just start knocking like crazy. Often, it just gets quiet again for no reason.

I've heard--but not yet verified--that White Industries makes a great freewheel that won't knock, but it costs way more than a Shimano or ACS. I'll probably get one when I upgrade eventually, but until then, I just live with the noise.

http://www.google.com/search?btnG=Go...ries+freewheel