When I first startup the car, I get a knocking about every second for 30 secs to a minute. I guess it could be an actuator or something. Any clues, any tests?

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)

Just guess here, but it sounds like the foot well flap motor lever. There is a test in SDS that will tell you if the motor/actuator is out of range...

Fairly common issue on W203/W209 cars

MB sells them as a improved kit 203 830 00 33

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