I’m in a 2001 Mazda B2500 and have a mysterious hissing sound behind the dash. The sound takes a moment to appear after startup, disappears when the A/C is turned on, and takes a moment again to reappear when the A/C is turned off.

Nothing else seems to influence the presence of the noise. I can’t tell if it is something rubbing/scraping or if it is air-related, such as a vacuum leak. The A/C seems to work fine and will blow air properly from the expected vents when changing the settings.

I can’t hear it under the hood. I’ve looked for leaks in the vacuum lines and poked around behind the glove box but I haven’t found anything obvious. Any ideas? Maybe it is a normal sound but I don’t recall ever hearing it until recent times.

Trying to fix all the little “quirks” the truck has acquired over time, so maybe I’ve just been paying closer attention and am noticing things that were always there...

 

Mine does this too. Only when turning or or off and only for a second. Sounds like something inflating. Some of the vents in the climate system are vacuum controlled, theres a vacuum switch and a few hoses to vents in the dash. I always assumed it was a noisy diaphragm or switch in the vacuum system. Maybe they all sound that way?

 

In a Ranger there are 3 vacuum "motors" in the dash, 2 operate the Vents, defrost, panel and floor
A 3rd, by glove box, operates the fresh air vent, its normally only used in MAX AC setting, it closes off the fresh air vent so the fan pulls in Cab Air that's already been cooled, so you get "maximum" AC cooling

Diagram here: https://www.explorerforum.com/forums...iag-gif.41768/

There is also a vacuum "motor" in the engine bay on the heater hose by-pass valve, its also only used in MAX AC, it stops hot coolant from circulating thru the heater core
Under the glove box on far right there will be two vacuum hose that pass thru the firewall to the engine bay, one is a black line which is the vacuum source from vacuum reservoir in engine bay
The other can be white or grey, and runs to the heater hose by-pass, called the Water Valve in diagram

Looks like this: https://i.imgur.com/AHlTteI.jpg
Thats the inertia switch just below it

The Heat Control(temp door) door is electric on 1995-2011 Rangers not vacuum operated

ALSO under the dash just to the Left of glove box door is the vacuum manifold connector, looks like a wire connector, but its all vacuum hoses, seen here: https://farm5.staticflickr.com/4205/...ecbbfda8_z.jpg

On the back of the Climate Control panel is the vacuum selector valves with all the vacuum lines, they run to this manifold, to connect Climate control panel to Trucks Vacuum motors and lines
This makes the system more plug and play for assembly and options

So there are a few places you can get a small leak, a hiss noise

And a vacuum motor itself can leak and make the hiss noise when its on