2005 Ford Escape Hissing noise under the dash

Brakes problem
2005 Ford Escape 6 cyl Two Wheel Drive 59,000 miles

When driving begin to hear a hissing noise under the dash, seems to be above the brake pedals. When you step on the brake, the noise disappears, then comes back a bit later, but at higher speeds I can't hear it.

Just had mechanic look at it and said they vacuum tested the booster and is holding vacuum. But the noise remains - please advise.

Thank you