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Discussion Starter #1
2004 - Vulcan - 205K
I posted this once before and no remedy. Now it's getting worse.

When I start the car from cold it starts normally and idles at about 1200 RPM for about 30 second then the idle slows and that's when the hum occurs. It is a "musical" hum and loud! You can hear it from 50 feet or more from the car.
It is coming from the intake somewhere and there's a strong resonant vibration if you put your hand on the intake bellows.
When idle slows to normal it will stumble and hum a few times and the idle speed fluctuates then settles down to a normal idle with no hum.
If you load it at idle, turn on the A/C or put it in gear or both, it will hum and stumble, idle is irregular again.
Under driving conditions it goes away if you accelerate, comes back if you decelerate. Will hum and idle rough intermittently.
Improves as the engine reaches normal operating temperature and drives more or less normal with very occasional hum.
It seems to be some type of intake problem but not necessarily a vacuum leak.
No CEL, at least not yet.
Installed new air filter.
I don't think a smoke test will help as it's intermittent and occurs while running..
Trying to diagnose with a stethoscope but difficult when it is intermittent.
Best guess is it's coming from the intake bellows area but not certain at all.
I tried to record a video to post on YouTube but phone mic doesn't seem to record the frequency well and can't hear the hum on the recording.
Any help with diagnosis would be much appreciated Thanks, John Kenneth
 

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X2...IAC and passages would be one of the first things I'd be inspecting and cleaning. Be advised that cleaning an IAC doesn't necessarily fix it. Only use a Motorcraft brand if you do replace it.
 

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I had a low droning noise on my 2006 at cold idle and it got quieter once warm and idle dropped. A new IAC valve solved the issue. Since I had three Taurus I just switched to test it and the noise moved to the other car.
 

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Years ago I had a howl from the upper intake on my 05 Tec. Sounded like a moose howl. A Mechanics stethoscope isolated the noise to the IAC. Cleaning / lubing the IAC made no difference. A new Motorcraft IAC and the howl was gone and hasnt reoccurred to this day.
 

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Discussion Starter #7
Hmmmm . . . "Low droning" and "Moose howl" I think we're zeroing in on the problem, especially two experts with 14,000 + posts between them! Although I have seen moose and eaten moose, I don't think I have heard one howl but I have a good idea how it sounds now. I have another Taurus and it doesn't howl. I will swap the IAC valves and see if that solves the problem. Thanks so much, John Kenneth
 

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Discussion Starter #8
Problem solved! The IAC took care of it. Idling fine and not a sound! The hum is gone, I guess the Moose moved on! Thank you to all who contributed, John Kenneth
 

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Discussion Starter #10
Can't figure what happens to the IAC to cause it to make that noise! Maybe valve plug vibrates after the it wears a little on the shaft.
The one that came off is entirely black inside, seems to be coated with black residue. The sound is some kind of resonance, the whole intake boot vibrates strongly when you put your hand on it. Like you say the valve/plunger thing probably wears. Action is probably supposed to be damped and maybe when it wears and leaks it vibrates at the resonant frequency of the intake system . . . or something! It lasted 206K miles so I don't mind replacing it.
 
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