Today after a few hours of riding two up I noticed a persistent howling noise from the rear of the bike. It sounds like wooooh woooooh wooooh when we are slowing down. The noise occures while slowing down only (braking and not braking), not accelerating. I dont hear it at high speeds but could just be the wind covering it up. I have heard this noise before while riding two up; just about since the day I bought the bike ('11 HSTC) the bike only has 7K miles at this point. Would appreciate any advise.