Do a search on "closing assist", it will give you lots of material.
The pump is activated by an electrical signal from the first catch on the door lock. It then runs untill it sees a signal from the door that it is closed, or untill the timeout circuit or the high pressure switch shuts it down! If it can not do the closing assist in three tries it disables that door or circuit. It can be reset by removing fuse#9 in the trunk. The hiss is an indication the pump is resetting the diaphram so it will be ready to "close assist " again. It is normal for the doors to pull in and then relax slightly as the pump turns off.