For several years my 1996 220 C (72k original miles) has "whirred" for about 10 seconds when first starting the car in the morning. It starts after the engine is running by quickly ramping up then after about 10-15 seconds would stop by winding down (as opposed to stopping instantly). Now it is doing it for a minute or even longer.

I always thought it was probably like a vacuum pump creating the initial vacuum or something so didn't pay much attention but now I am getting concerned. The sound is like a cross between a whir and a whine.. quite high pitched. (sounds like a cross between a vacuum cleaner and a dentist's drill)

I tried to locate it under the hood without success. Does anyone have any idea what this noise is and what it means that it is lasting longer. I also noticed it reoccurred today while driving in town about 4 minutes after starting... for just a few seconds.. then stopped.