The noise you're describing sounds like a collapsed piston skirt. A loose rod will sometimes turn into a double knocking noise when the ignition is disabled, while a piston slap will almost disappear. You can drop the piston down far enough to change the rod from the bottom, but you won't be able to remove the piston that way.
Drop the oil pan, pull the head, and push the noisy piston out. If there's enough ring ridge to hang a fingernail at the top of the cylinder, ream it out before you try to remove the piston. It might be a very good idea to check the rod bearing clearance before going to the trouble of pulling the head and the piston. Anything over about 3 thousandths is too much clearance on the rod bearing.
Jerry
