-possibly your axle nuts are loose?
-brake pad has broken off the rivets, and is just slapping around inside the caliper
-wheel bearings (dont usually go out at the same time)
-loose lug nuts?


worst case scenario is that your tranny is going to shit, but thats dosent make sense that you only hear it when your braking? that would happen all the time.


get in your car, drive it about 20 mph, and put it in neutral, if the noise is still there...its not your tranny.