I would say that its your tranny. The fact that the sound stopped when you added fluid proves that, I think. There has to be a leak somewhere since the fluid usually doesn't burn up that much to make it grind. Put some cardboard under the car and see where its coming from. Even if the noise doesn't come back, you'll want to keep an eye on it because any grinding noise coming from the tranny is definitely not good. Might be okay, but if there are any more problems then take it to a shop.