my mates's primo flipflop hub makes a noice like (and i quote) "a cat crawling around on a glass window, howling like its seeing death first hand in slow motion". any ideas why? he swears blind that the axle is fine and that there is no dirt in it. he won't let me check it out, even though i am known to be a competent bike mechanic. any ideas on what the source of the noise could be?