All,

I have an '04 4.0 sohc explorer that has a hissing noise under the passenger side dash each time the throttle is pressed (hissing last for a few seconds). It also makes the same hissing when the AC is turned off and on or the truck is turned off. Each time it only last for a few seconds, like something is equalizing, but it is annoyingly loud. Anyone else have this

 

I have the same explorer with 38,000 miles on it and just noticed that mine is doing the same thing. Did you ever find a solution to the noise?

 

My f150 does this, I believe it's the vaccuum lines/diaphragms that control the heater/ac. No real solution except for leaving your heat/ac to off, or turn up the radio

 

find and fix the leak.

 

It ended up being the vacuum reservoir, it's that cylinder under the dash that has a vacuum line with a T connector on it. I wrapped the cylinder in sound deadening foam and covered up the end real good where the vacuum line attaches. Note that my problem (ie noise), though present when turning on and of the ac or heat, did not only happen when the ac or heat was on, like some have mentioned, it also happened every-time the throttle was pressed, truck was turned off etc.

Hope this helps,
Scott R.

 

why did you not replace it?

is it still leaking?

 

my does not leak and has hissed like that since new.

 

There is no leak, the hissing is from the air moving in and out of it, the location just makes it real obvious in the cab, if it were tucked under the dash, I would never notice it. Why some do it and some don't may be slight difference in internal config or flash or something, so there is no way to know if the next one would do the same.