I think he has the same sound that mine makes occasionally. Same type of symptom; start car, foot on brake, shift out of Park into Drive, let foot off of brake, hear a short sound coming from up front somewhere. It almost sounds like a musical tone, like if you rubbed the rim of a crystal wineglass, for just a second. I've had it since I got my car in 1999, it doesn't always do it, but it often does it after the car has been sitting in my garage all night. I like Bob Dillon'* suggestion to lube the parking brake cable, that might be it. I dunno. I've had my dealer check it out before and they've never found anything wrong with it. I've had pads replaced before and had no change in the sound. I have no idea what it is, but I just chose to live with it. In seven years of ownership it hasn't resulted in any component failure (yet).

I'll put in another vote for checking the vacuum booster, check valve & hose.