You keep saying "freewheel". Is it really a freewheel? I have an old Sun Tour ultra six freewheel and sometimes, but not always, when I'm in the 3rd smallest cog it will clunk-clunk-clunk continuously as I pedal. If I just backpedal a little and then start forward again it usually goes away. Doesn't do it in any other cog and doesn't do it every time. I think it has something to do with the bearings being slightly out of adjustment, but I'm not sure about that and so far haven't felt like trying to adjust it. Hope this can be of some help.