Can you make a recording of this sound? This might seem like a lot of trouble but it could help you reach a correct diagnosis in record time!

If not, on warm engine - so after at least 20 minutes of driving - rev the engine at stand still to see if you can reproduce the problem. You say it is noticeable - I think you should be able to hear something going wrong.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!