My 260E (290,000 miles) does this every morning for a few seconds. I asked numerous mechanics who simply told me that it is cold and for the few seconds until the oil is pupmed upwards the knocking is caused (not hard knocks but rather light) and not to worry about it at all. Actually my car does not use any oil and I just drove it to Los Angeles and back over a 2 week period and we did it without any problems (4000 miles).
Igor Zeljic, Minneapolis.
1984 MB 300D Turbo Diesel W123 Sedan 132K
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2002 Audi A6 3.0 Quattro Avant 49K (Wife's car with warranty