My Yamaha YBR has passed it's MOT and had a service recently but something that has come up since then which is a loud droning sound from the back break when slowing down when I'm 30mph and above. I'm not putting my foot down on it but at a certain point it will make the sound so I ease off and apply more front break to make up for it. It's not a high pitched squeaking sound, it almost sounds like a horn, it's a dull hollow type of sound. I was wondering if I was using the back brake the wrong way. Would this sound familiar to anyone here?