Try taking a paper towel core and hold it to your ear.

Locate a general area or the location as it might be beneficial to us to come up with possibilities.

I almost want to say it's in the area of the brake booster, a cracked hose or the face of the check valve.
Crack appear on the under side of most hoses. They do this just so they can be difficult to find.

Whoops, guess that was more than almost!

Phil