Friends,
I have a 96 SL320 that I got about 2 months ago, great little car, had some cosmetic problems, but mechanically very sound. I have noticed that when I start my car and the engine is cold that I hear a loud noise coming from the engine compartment, I think it originates from a device mounted near the alternator that is not belt driven, but has a wire connector in the front of it where you would think a pulley would be. I am thinking that this is the Vacuum pump that is driven by an electric motor? and when the car has been sitting for awhile that the pump activates to build vacuum in the accumulator? Let me know if I am correct or if I need to take the thing in and have it diagnosed. Thanks for your help and have a great day!
Regards,
Brian

Thanks again Dr. D!

Yes, that is the vac pump. It will run for 30-90 seconds every time you start your car and cycle off/on from time to time when driving. It is used to help heat the cataylist and o2 sensors up.
If it were acting up, the Check engine lamp would be on.
Dr.D