I would replace the plug on principle. The miss may be caused by a badly fouled plug, or a bad plug wire or possibly a cam or rocker worn so badly that the valve is not opening, or all of the above.

The best way to test this IMO is to attach a inductive timing light to the wire, start the engine, and see if it flashes. I would est all the cylinders this way.

As noted, the noise could be a bad ball stud, or worn cam lobe/rocker arm. Use a 2 foot piece of hose or an engine stethescope to see if you can isolate it to a cylinder, then pull the cam cover on that side and have a look at the valve gear. Do a search on "ball stud" and you will find a lot of information on how to check them.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe