Whooshing sound when braking to a halt

When braking, noticeable more when coming to a halt at slow speeds there is like whooshing sound.

When I took the technician out he thought the brakes pads might need resetting (i've done about 1000 miles so far).

When I picked the car up I was told that what I was hearing was the booster vacuum release and that this was normal. What is the booster vacuum release and is it normal as I've not come across this before.