That is pretty much what it boils down to. Pull the valve cover, and check to make sure the plug is there. If it isn't, you've likely found the problem. If it is you've ruled out that possibility and the prognosis starts to get more grim. Bottom end problems are very rare but I have seen a 119 with bottom side problems and the diagnosis came after ruling out all other top end possibilities. It sounded a lot like a lifter noise but it was a piston slapping hard with no skirts on it. Hopefully that isn't your situation.