Hmm, it *sounds* like you have a vacuum leak and your vacuum pump continues to run, trying to get proper pressure to unlock the doors. I say this because you say it's coming from the trunk area, and happens right after you unlock the doors...
I would get in the car, be very quiet, and lock, then unlocking the doors, then try and find a hissing sound (like air rushing) to see which of the door actuators might be leaking. Are you having any other problems with your door locks? One not unlocking, persay, etc., etc.
2006 E350 w/ 155k miles (Daily Driver)
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!)