The 60x engines have hydraulic tappets. If the car sat for a long time, they can drain out and be noisy. Usually they'll pump back up in 10-20 miles of driving, but sometimes they don't if there's a bunch of varnish or sludge in them. They can be taken apart and cleaned (I've had good success doing this on 3 different engines) or replaced with new (parts aren't that expensive). Try a 5W-40 oil and drive it on the highway for 30+ minutes and see if it shuts them up.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]