Bumping noise when braking in reverse

This is in regards to a 2001 Jetta GL 2.0 with about 45,000 miles. I started to hear a bumping noise this morning when I was braking in reverse. I think it sounded like it was coming from the front drivers side of the car (I was in the passenger seat). But my boyfriend thought it sounded as if it was coming from the rear (he was driving). I am going to change the brake pads this weekend and I will be getting new tires soon, but I don't think that those things have anything to do with this noise. If anyone has any ideas of what this could be, please let me know. Thanks a bunch!