When I release my brake pedal from a full stop I hear a hissing sound under the dash. It sounds like air is leaking from somewhere. The weird thing is that if I tap the brake it goes away!! It only makes that noise when the brake is released from a full stop and then to stop the noise I have to tap it. What the heck is this?? Anyone have this problem before?? I have an 03 expedition 5.4 xlt

 

Disconnect and plug the large vacuum hose going to the brake booster. If the hissing went away you'll need to replace the booster.

 

Leaking booster

 

Thanks for the replies. The only problem is that it's not doing it today. Is that something that goes and comes when it's going bad??