Is the neck stem loose. Put the bike on the center stand (assuming your talking stock bike)........ Put the weight in the rear....... May need a buddy or a bucket n rope. And jiggle the front race stem if your stem bolt is the tiniest bit loose that will cause your front end to bounce too... Or possibly even the back tire way out of alignment..........Spokes loose? Visible dent in the rim............Sometimes you have to look really close. I've bought two bikes that had hit bumpers or curbs that I didn't notice at first.Till I had them in the shop for closer inspection.