"HISS" Vacuum leak ?

[dogs silver S60]

Hi all,

2001 S60 150,000miles.

Just started yesterday. When I RELEASE the brake pedal from the depressed position, I get a high pitched Hiss, whine sound.
It seems to be coming from very close under drivers side dashboard. It stops after 15-20 seconds. I'm thinking vacuum leak. Wheres the most likely spot I should look? Brake function is fine but wife is freaked. Appreciate input!

Thanks. Brian

[jda2000]

your brake booster servo has failed, you need a new one.

There is another post going on now about it with instructions, on how to do it yourself and save big $$$$.