I have a 99 S320 and have no such noise.. There is a blower under the hood that sorta sounds like a whine during the first couple of minutes after startup but otherwise as quiet a car as I have ever owned.

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Bill

Backyard Shadetree Mechanic, 30 years running.

1987 300TD
1988 300E (sold, sniff)
1999 S320
1981 Volkswagen Rabbit Truck (Diesel)
2001 Ford Expedition
1988 Suzuki Samurai
On my list to buy: R129, for me, and a R107 for my wife.