I once thought I had valve noise on one of my cars. Turned out it was the smog pump. Remember, the smog pump runs for about two minutes on a cold start. You would expect to hear lifter tick/valve noise in the first minute after a cold start as well, hence my misdiagnosis.

Try disconnecting the pump and see if the noise is still there.

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles