Could be the auxillary water pump -- this time of year it runs a lot.

Need to check the current draw if you can hear it -- if it locks up it will fry the pushbutton control unit.

It could also be the interior temp sensor fan -- if so, you hear it because the foam hose under the dash has fallen apart and needs to be replaced. Temp control will be iffy if this is the case -- swing hot, then cold, then hot, etc.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!