Only at night? Probably because of the air pressure difference (??). It is probably happening all the time, just more pronounced at night time.
One of your door lock (or trunk) actuators is probably worn (has a leak in it). I currently have the same problem in one of my doors.

I am not sure if your trunk lock is electrical or vacuum... I know they changed about 3 or 4 years ago, but I'm not sure if it went to electrical from vacuum, or vice versa?

Do what I suggested earlier and try to see if you can hear a leak inside the cabin.

One more thing, is it much more pronounced on unlock, meanwhile, locking, is fine? I have the same problem, and this is because when it's locking, it's sucking the actuator (rubber, I believe) together, thus "sealing" the leak, while when opening, it blows all the air right out of the actuator, thus the continuing run-on of the vacuum pump (buzzing sound), in an attempt to equalize (or whatever it's doing) the pressure. But it will eventually time-out (shut off) after a certain amt of time.

Keep us updated! AND GOOD LUCK!!

__________________
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!)