You have to take the top off the tank. Then take the turret off the top, then take the whole turret apart and remove the barrel.
about as far of a tear down as is possible. Sorry.
maybe take a long rod, hold the tank so it faces down and try ramming the bbs and see if the will fall out.
As for the noise, if you can’t find it keep driving. You will either find it or it will go away.