flat spot on a rim,maybe two or three spokes wide?,check the rim run out and the spoke tension,if you can tighten any dull sounding spokes do it evenly,tap each one and make a texta(sharpie)mark on the dull ones,then try to tighten them a little at a time evenly,if a few are real dull do it with the tyres deflated,if the spokes will turn that is.