is this on your 97?
it should be silent in the cabin nearly so anyway. did you check your powersteering fluid? does your car have SLS? did you check your sls fluid? is there any oil in your engine compartment signifying a leaking hose of some kind?
there are cams and lifters in the valve cover. you will hear slapping/clicking with the oil fill cap off... normal. how's it sound under the hood with the cap on?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!