Engine hums when below 1000 rpm, code P0174

2000 MERCURY MOUNTAINEER
195,000 MILES • V8 • 4WD • AUTOMATIC
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SHENNY FERBRACHE
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Recently we had to go on a couple long trips, each 600+ miles round trip. On the second trip, we noticed that whenever we slowed down and the rpm's dropped sharply or dropped below 1,000 rpm's, the passenger side of the engine has a loud hum. It almost sounds like a conch shell being blown or a note on a low brass instrument. If we pop it into neutral and keep the rpm's up, it doesn't do it. It also tends to do it less when the engine is cool and/or we keep it below 15 mph. The check engine light is on, it has a code of P0174. We cleaned the MAF, just to be sure, and also all the fluids are good. Does anyone have any ideas? Its really loud and obnoxious, and in a couple weeks we're going to be doing an 800 mile round trip, and we don't want to break our only car worse. Thanks everyone!
Jul 9, 2019 at 2:59 PM
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KASEKENNY
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Hi,

I can't say I have heard this noise before by the way you described it but is there a chance you can get a recording of the noise and post it then maybe we can assist better? Thanks
Jul 9, 2019 at 3:45 PM
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SHENNY FERBRACHE
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I tried to get a recording of it several times, but my phone mic can't pick it up. My husband's description is the sound a glass bittle makes when you blow across the top, like a low flute sound.
Jul 9, 2019 at 4:44 PM
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SHENNY FERBRACHE
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The rig is an AWD, if that matters.
Jul 9, 2019 at 4:55 PM
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KASEKENNY
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Unfortunately, I am not going to be able to tell for sure what this is without hearing it, but I can tell you how to find it if it is my best guess. When this is acting up are you able to get out and listen to specific areas? So will it continue to make the noise if you are stopped in drive and foot left on the brake? If so, have someone else drive and get it to make the noise and then pop the hood and inspect the IAC. This is the idle air control valve. This is the most common thing that makes noise on Ford/Lincoln/Mercury engines. In fact, refer to this guide step 6 and it mentions this.

https://www.2carpros.com/articles/engine-noises

Please let me know what you find and I will be happy to brain storm some other ideas. Thanks
Jul 10, 2019 at 3:52 PM