That rumbling may just be the pad backing plate rubbing agaist the oxidized (rusted) portion of the rotor. You know, where the pad didn't keep the rotor nice and clean/shiny. The backing plate is wider/taller than the pad's friction material, so when you don't have any more friction material left...you backing plate will rub up against the highest portion of the rotor surface.
This sounds very similar to how my rear brakes ended their life. I would suggest getting your pads replaced ASAP, before you damage your rotors and have to replace them too. If you are on a tight budget, find the cheapest pads you can get (generally ~$20/set) and throw them on until you can do the job fully. While this isn't the "best" thing to do, it can extend your $$ outlay a bit.
