This sounds like the problem I was having with my 1985 500SL. The tapping/knocking sound was most prevlent at start up, but soon became less pronounce. It turned out to be a worn cam lobe and rocker. With the hydraulic valve lash adjusters, this excessive lash will result in noise until oil pressure to the adjuster takes effect. Of course, loose big ends or mains could cause similar symptoms, but low oil pressure would also probably be a problem as well.
w198 300SL 1963
w107 500SL 1985