Getting ready to have the 12k service on my 08 classic lt ,I have noticed as of late a vibration in the floor boards (like rolling over a short rumble strip). Thought it may have been the brakes, but during a recent tire change out I asked the mech. about the brakes he said they were fine. This is not constant just at very slow speed or just before a stop. I know this is a lousy description of the problem but , any ideas?