After you check the crash bar mounts to make sure they are not loose, Try moving the pegs up or down your crash bars a few inches, and see if that does anything. Or try removing them completely. If moving them up or down helps, perhaps look into the possibility of using offset mounts, so that the mount is clamped to the bar at a different position, but the peg is where you want it.

I seem to recall reading a thread where the guy discovered that if you mount the pegs too close to the lower bend in the crash bar, you will get the vibration.