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Hissing sound??


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February 27, 2011
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Can anyone help out or give me some direction here?

Picked up a 2007 ST XLT....4.0L/V6, yadda yadda...

there is a small hissing sound when I accelerate but it goes away unless I accelerate again...the sound is coming from the passenger side, near the floorboard or even the side wall (kind of where the fuel reset thing is)...

I also notice it when I turn the truck off...

I want to say it's a vacuum leak of some sort, but thought I would let you all chime in to see what you have to say...anyone else have this experience also?

If so, what was the problem and how much to get it fixed???


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Do you have a CEL on right now?

Is it under heavy acceleration or light?

Is it under heavy acceleration or light?

All acceleration, minor and heavy.

Like I said...it tapers off as the speed levels off, but if I were to increase the acceleration I would hear it again.

Mine did the same thing . There is a small vacuum check valve in the supply line for the system. It is located in the engine compartment. Look for a hard plastic line coming from the r/s of the firewall area leading to a vacuum source on the back of the intake manifold. In that line is a one way check valve . It sounds like yours is stuck.

As long as its not real loud and doesnt sound like a leak, it's completely normal. It should just sound like a rushing air type of noise inside of a canister. Ive had multiple 4.0 explorers and my 08 and 04 both did the same thing. It is fine.

Curious , does your sport trac do it ?

i have a 4.0 and recently installed an aftermarket throttlebody....heard the hissing sound and i assume its because of the increase of volume of air going into the engine...same thing it goes away after the rpm levels...but it hasnt caused any problems of sort....so it doesnt bother me

My 4.0 has the small hiss/whistle on light throttle until about 3k. I don't mind it at all. I kind of like it, reminds me of the turbo diesels.

well my 08 ST just started doing that today so I thought Id search here and see if anyone else is having that issue. I have the 4.6 and its doing the same thing as Velo mentioned. I'll check the vacuum line in the engine compartment as soon as my wife gets back, she just took it shopping. hopefully its that simple. Im assuming the check valve is replaceable/servicable pretty easily?

Mine does it too. Sometimes it's louder at idle when I first start it. I don't drive it that often so I'm not too worried about it. I just tell the wife it's secret boost under the hood.

Try pulling the throttle body and giving it a spray with cleaner. Mine was a city truck, and carboned all to hell. It was causing the acceleration whistle in mine. I also cleaned out the MAF sensor while I was at it and got another mpg or two.

4.0l I must add.

Well I found this check valve that's in line a vacuum line that goes through the firewall. The check valve was leaking by. Hopefully that's it, but ford wanted $45 for the valve that includes the lines with it, Wtf