I had the same problem on my 300SEL 89 a couple of weeks ago. I had a post but did not receive any reply. The problem was for first 2-3 secs the car would sound like a diesel only after a overnight start.

Guess what changed the oil and switched to 91 octane fuel and I have not heard that again. Just my experience.

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SANDEEEP
89 300SEL
230,000Km