My 1995 Ranger is running well but lately I have notice a noise that sounds like hissing air either sucking or blowing when I let off the gas pedal while the engine is accelerating. It seems louder the more strain the engine is under prior to letting off the gas. It’s a very quick hissing air noise that sounds like it’s coming from the back of the engine near the fire wall, but that’s just a guess. Any ideas, could it be an exhaust leak? 1995 XLT 4.0 4WD.

 

See if it changes when you lightly press on brake pedal, there is an air valve in the cab inside the brake booster, the atmosphere valve

If you have cruise there is also an added valve in the same place that releases vacuum from cruise module in engine bay, so a vacuum line that runs thru the firewall

When you let off on gas pedal the Vacuum in the intake goes very high, normal vacuum at idle or cruising is 18"-21", it can go above 30" when decelerating
When accelerating it drops to under 10", if you were to let off gas pedal at that time it would pop up above 30"

Vacuum is all in the intake manifold, but its also used for power brakes, cruise(above), PCV system, EGR system, Cab Vents(defrost/panel/floor)

 

Here are a couple of videos, one is of the brake being pressed and released, which sounds like air is coming from the piston valve behind the brake pedal. The brakes seem to work properly so I’m not sure if it’s anything to be concerned about. The second video I had to be driving the truck to produce the issue, so I just held the phone under the dash as I accelerated and released the pedal. As I let off the gas you can hear a hissing sound similar to the one in the first video but not as obvious.

 

When you press the brake pedal air SHOULD rush in to that valve, that's how power brakes work, the engine provides a vacuum inside the on one side of the booster diaphragm, when you press the brake pedal down the "outside" air pressure(15psi at sea level) rushes in and assist you in pressing down on the pedal
There are usually filters in the valve so you won't hear it too much

Not sure what we should do with the videos????

If you hear a hissing noise like air escaping or being sucked in then look for hoses under the dash
Only hoses I know about, as mentioned above, are for the cruise control and Vent "motors"(do all the vents work?)