Have you checked the poly V belt pulley bearings, you will need to remove the belt to test them but as it rev related I would supect something in that area.
Have you managed to get MB or a garage to listen to the noise, I certainly don't like what I'm hearing if you have isolated the water pump, alternator, air con compressor pulley bearings which you did by removing the belt then this is something more serious, any signs of burning oil, water in the oil , oil in the coolant reservoir? problems starting?
Well there has got to be reason why the noise is there but sorry i can't help, how long ago since you changed the oil? as your aware normally the 'a class oil stays pretty clean is yours OK.
Even though the car is running OK I would try and get it checked out you obviously realise this or you wouldn't have asked for help.
Vehicle: Mercedes 500 SL from 1991, A140, Volvo V70
Location: Poland
Posts: 45
when i bougt this car engin oil was very bad and the noise was less noist becouse oil was very sticky. when i change for mobile one 0W-40 the noise was louder..
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