I can imagine that has something to do with the vibrations of the sound on your teeth. The ear canals and the troat are very closely linked by the eustachian tube. The eustachian tube connects the ear to your airway. This is why sometimes when you have a sore throat it can almost feel as if its also deep inside your ear. It is also the reason our ears pop when we yawn. The eustachian tube has to open up to equalise the air pressure. The vibrations of the sound waves can easily make their way into you throat and mouth. These may cause i tiny vibrating sensation on your teeth which will be felt more strongly when the bristles touch as the vibrations move to the bristles. Whats really cool is sometimes when you open your mouth,if it's quiet enough other people can hear the music.