I just bought a 98' Mustang GT and im hearing this weird noise from the rear right side. Like a grawling type noise a mechanic told me it could be my wheel bearings but i thought wheel bearing only make noise when turning....i hear this noise when im on a straight road and faster speeds i hear it more and and when i break it slows down and is not as loud. what could this noise be from?

Any help would be great im kinda new to the whole mustang world

thank you