I just noticed yesterday that whenever I put my truck in reverse or drive I hear a brief grinding sound. The truck drives fine, but why am I just hearing this now?

I very recently changed the tranny fluid, which is still looking a real good red color, and isn't low. It looks like I actually had over filled it, but I'm not sure how to drain tranny fluid without popping the whole filter case. (A mechanic helped me with the fluids change, and wasn't concerned, I don't think that is the problem, but it is worth mentioning).

EDIT: It is more of a metal bending/squeaking sound than grinding. It lasts about three seconds, and once in gear its gtg, and drives fine. Is this just something normal, like rust shaking off, or am I doomed for transmission failure?

 

Nevermind, I got it. The rear brakes make the grinding sound. Not sure why, maybe it is some rust. Or it could be the emergency brakes, which do not work. Perhaps I should fix them sooner than later.