My guess: Oil cooler.

You said not all the time. I believe the Oil cooler is thermostatically controlled ....

..... or the rear suspension self leveling pump?

Are you loosing engine oil?

BTW, have you checked your power steering fluid level recently?

Haasman

__________________
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)