The PSE pump is under the right rear seat. It makes a pumping sound each time the doors are locked or unlocked. One side is pressure and one vacuum (lock or unlock). It could sound different for each. Sit in the car and lock and unlock the door and listen for the pump. If you can't hear it then you probably won't hear it as I surmise.
It is the location of the noise and your description of the sound that draws me to the first impression. That is just the way I would spell the noise a pump makes.
Bosch Master, ASE Master, L1
33 years MB technician