On the later W202 cars the in-car temp sensor next to the sunroof switch will make a constant noise that is very annoying.
The sensor has a built-in motor that pulls air across the sensor.
MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)