if the ac pump has not been changed then that would be my main suspect, sometimes they make noise intermittent, sometimes they make noise all the time, sometimes they lock up and kick the belt off. I would check it more thorough in case it is bad and before it throws metal thru the whole system. you may be able to hear it better if you get under the front right bumper area and have a helper rev the engine while you listen, needless to say don't stick your head or fingers up in there while the engine is running
if you hear what sounds like little bits of metal being chewed up, then take the ac belt off and have the helper rev the engine again to make sure the sound is gone.
