Well, before I broke mine the other day (stupid, stupid, stupid...) it always did the same. The sound you are hearing is the air releasing from the other line. While trying to rig something up until I get a new switch, I found, that when you move the switch, and it directs the vacuum into another hose, the first hose releases it's air to allow the other hose to do it's job... So, basically, the air that is used to hold the headlight doors down, is released, so the air that is used to hold them up doesn't have to fight over the air holding them down... Atleast thats what it looked like to me, because when I didn't release the other air, the headlight covers looked like they were fighting themselves to come up.
Hope I made some sense...