Thanks for the input guys!
It sounds like an issue I should look into soon. It might be something I could do myself (if it's the rockers). Need to get the MB CD-ROM shop manual. Does that, by chance, have the procedure for rocker-arm replacement?
Will have to wait for warmer weather (another 2000 miles of driving). Hopefully it won't be too late -- that is, needing to replace the cams etc.