Don't know if the symptoms are similar, but another C43 owner here in Charlotte told me that he "had cams replaced - always noisy above 2k rpms. Had lifters replaced to better hold the oil on top of the engine after it sits for a few days. Both fixes corrected the problems."
This was done under warranty -- I would think that the individually matched AMG camshafts would be very pricey.
'98 C43 AMG
'93 190E Limited Edition (including factory Sportline package)