Howling noise when decelerating

2001 FORD F-150
210,000 MILES • 4.6L • V8 • 2WD • AUTOMATIC
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JOJOSTREET
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When letting off the gas, there is a howling or horn noise that is sounding from the passenger side. Even does it when braked to stop. It stops when pressing on the gas. It happens more in colder weather. It does not do it every time. I can also make it quit by pressing back on gas when slowing down. It is loud, People inside my house said they can hear it. When it first happened, I thought it was a fan in the dash, but it was not on. So it has to be under the hood where sound resonates on passenger side. But outside the cab seems to be loud if I am outside and people inside house can hear it.
Nov 30, 2018 at 6:22 PM
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JACOBANDNICKOLAS
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Hi and thanks for using 2CarPros.

If it is possible, upload the audio so I can hear it. Also, let me know if you hear it with the fan motor off.

Now, the HVAC actuator doors are run by engine vacuum. When you press the gas, engine vacuum decreases and therefore, the noise goes away. When you let off the gas, it increases and you get the noise. If I am correct, one of the actuators is leaking vacuum or one of the vacuum hoses is damaged or disconnected.

Craw under the dash and you will see vacuum lines. A supply will come from the engine to the control panel. From there, they will branch off to actuators. You need to see if any look damaged or disconnected. Also, try running the temperature the opposite as you have it, change air flow directions and see if everything is actually working or if one of the different selections changes the noise. There is an actuator for temperature, and two for mode (air flow direction). Any one of them could be the problem. If you find one not working properly or slowly, that is where we will focus.

I attached a general picture for you to see. It indicates where the different actuators are located. Focus on numbers 2, 5, and 8. Keep in mind, these things have vacuum modules which should hold vacuum. If there is a tear or dry rot issues, they will make noise. I actually heard them sound like geese. lol

Let me know what you find.

Joe
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Nov 30, 2018 at 10:24 PM
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JOJOSTREET
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At first you cannot hear it as I am going 45 miles an hour. When I released the pedal if you can see the the mile per hour around 35 you can hear it hum until I come to a stop. I turned and I was trying to put phone out window, but you cannot hear it.
Dec 1, 2018 at 10:57 AM
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BMDOUBLE
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I’ have had lots of idle air control valves cause this whistle/droning noise. You can either try to clean it or replace it.
Dec 5, 2018 at 6:10 PM