2003 Ford Expedition hissing noise

Yes, it sounds like a vacuum leak. It sounds like the brake booster is leaking or there is a leak in the hose that runs from the engine intake to the booster. If you can not see or find a leak, take the car to a nationally recognized parts store and have them to a vacuum test on the brake booster to see if it maintains vacuum.

Let me know what you find or if you have other questions.

Joe