Is it rattling when you shoot, or is something loose all the time? You said it was coming from the buffer tube, did you take it out as part of your cleaning? If not I would take it out and see if anything is in there. If you don't know how to take it out, just push in the small pin in front of the buffer and let the buffer slide out. Lock the hammer back so it does not get in the way when you have to pull the spring all the way out. Look to see if anything is inside if not, just wipe off the spring and dirt out of the tube and spray a light coating of oil in. Not too much or it will just attract dirt. If you have any problems just post it, I will check back later. Hope it helps.