I took a video showcasing the noise in question, here is a link to it, it's the consistent tone, not the engine strokes. https://dl.dropboxusercontent.com/u/20211780/Howling%20Car.mp4
It only occurs while Idling, any non-idle engine activity (Such as acceleration or turning the wheels back and forth while stationary) will cancel the noise until the engine returns to idle. The noise is not consistent in volume between stops (the video was taken when it was rather loud.) And sometimes fails to occur entirely.
The only recent change to the car was an air filter replacement about 2 days prior to when I started noticing the noise, though I did get a belt change the day after I started noticing it for other reasons. I will also admit I'm not always the most attentive, and a bunch of other noises recently got fixed so I might have just never noticed it.
Closest description I could find on the web was low Power Steering fluid, but my PS fluid is at the MAX mark and is not leaking.
My car has 270000 miles.
My car has an automatic transmission.
The power steering pump may whine if the fluid level has become too low from a leak or you may hear a squealing noise if the belt has become too slack. While the belt is off would be a good time to check the other components along the belt drive as well to possibly catch a problem before it gets too bad. Since the belt drive was serviced recently, it would be a good place to start looking for problems. Consider hiring an experienced technician like one from YourMechanic who can come out for a closer inspection of the noise to offer a more personal diagnosis.
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