This also could be a ball stud gone bad - these are the hydraulic gizmos screwed into the head that support one end of the rocker, and allow the rocker to ride on the cam lobe at all times. The fix is usually to replace all of them or at least the ones on the side making the noise.

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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