1999 Jeep Cherokee Hissing Sound when brake pedal is pressed?

This morning while driving, I began hearing this hissing noise as if air was escpaing from somewhere inside the car, the noise appeared to be coming from the brake pedal. When I press on the gas the noise went away, when I parked the car there was still the hissing noise but it was not as loud, turned the car off and the sound died down and went away.

What could this be. please help! Thank You!