I have been riding an older Trek with a circa 1992 7-speed freewheel setup on the rear and recently have experineced an intermittant tapping or ticking. At first I thought it to be a spoke, since the wheel is not really very well tensioned. But the sound would come and go and seemed to depend on how hard I was riding and in what gear. After a bit of poking about we found a slightly bent rear axle - the noise is probably due to the bearings being slightly trashed. Need to take it apart for the final diagnosis....