Unknown noise coming from engine bay while running

2001 FORD F-150
227,000 MILES • 5.4L • V8 • 4WD • AUTOMATIC
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VGO
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I just bought this truck a couple of days ago. It just started making this noise today. Every couple of seconds it will make this noise. I don’t really know how to describe it so I included a video. It isn’t revving, moaning, screeching or knocking so I have no idea what to categorize it with.


Overall performance hasn’t suffered nor is there anything out of the ordinary or any warning lights or rough gear changes.

I have someone that can help with repairs, so that isn’t an issue, but an unknown noise isn’t something to sneeze at. Any help would be very much appreciated. Thanks.
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Jun 14, 2020 at 5:03 AM
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ASEMASTER6371
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Good afternoon,

Remove the drive belt and see if the noise is still there. I am trying to see if the noise is from something belt driven.

https://www.2carpros.com/articles/replace-serpentine-belt

Roy
Jun 14, 2020 at 6:04 AM
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VGO
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I just spent some time testing: There is a new hissing noise in the cab when the A/C is turned on, like air escaping a balloon. This is also when the mystery noise occurs.

Turning off the A/C makes the hissing stop and the noise in the engine bay stop as well. Idling and driving without the A/C running sounds good, the noise didn’t re-appear.
Jun 14, 2020 at 6:14 AM
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ASEMASTER6371
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Then that noise is a vacuum leak. Some of the mode doors run on vacuum.

Look at the vacuum tank or hoses under the hood that feed into the truck for A/C controls.

Roy
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Jun 14, 2020 at 6:35 AM
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VGO
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The compressor is the culprit for the noise. Seems to be failing run at all.
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Jun 14, 2020 at 9:02 AM
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ASEMASTER6371
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Did you remove the belt as I suggested way back to confirm? You could spin it with it off and confirm the compressor is the noise.

https://www.2carpros.com/articles/replace-air-conditioner-compressor

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

Roy
Jun 14, 2020 at 9:04 AM
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