Sorry if this sounds stupid or not.. but I bought a 300sel on Friday and haven't had any prior experience with MB or any other car as the matter of fact.

My question is...
when I lock the door, I hear a humming sound. I found that noise comes from the trunk somewhere in that region. It only makes that sound when I lock or open the door, is this normal? if it is.. what is it and whats the function. if its not, what should i do about it?

thanks

This is the vacuum pump... it is what unlocks and locks the doors. It does this via vacuum lines to actuators in the doors, fuel filler door, and trunk.

All normal!

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

the above post by David Klasse is correct, it's the vacuum pump, and it's normal so long as when you lock/unlock the doors you only hear the buzzing for a couple seconds or so....if its like mine was and it went on for a good 20-30secs then you've got a vacuum leak somewhere in the central locking system and the pump is trying to make up for the loss in vacuum.

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D. Chang
Network Integration Engineer
MCSE, CLP, CCNA

Great, now I know its normal. What is the function of the vacuum? does it take air out of the car or what?

It sucks air through the vacuum lines (to each lock) to an actuator, which, via air pressure, unlocks or locks the individual door.... it replaces a "normal" electrical locking system (motorized locks).

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

I believe your car, like mine, has a central locking system that uses vacuum as the actuating mechanism....as you lock/unlock your doors, it uses vacuum to actuate the other door vacuum elements which unlock/lock their respective doors.
So when you unlock your driver side door, all other doors unlock as well and vice versa.

the vacuum itself, activates a diaphragm in each vacuum element causing it to either contract or expand depending on the operation involved, which moves an actuator rod connected to the door locking mechanism....

__________________
D. Chang
Network Integration Engineer
MCSE, CLP, CCNA

Great, thanks for the explanation. Learned something new about my car