Impala moaning sound?

I know this may sound funny, but my wife picked me up for lunch today and commented that the car was making a moaning sound. I laughed, but sure enough, at the first red light, I heard this low freq moan. It
moaned at idle wether in park, or drive, but any slight RPM above idle and the sound disappears.
Car is a 2001 Impala LS, 3.8, 55K miles.
So, she parks and shuts it off. I pop the hood and see nothing wrong visually. I ask her to restart the car, and no moan?
She said for the rest of the afternoon no strange sounds?
Any suggestions?
Thanks, and have a nice day,
Bill
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billccm wrote:

Did you look under the car for any errant pedestrians? *lol* Sorry, I couldn't resist. Brakes will sometimes "moan" when you are stopped and you ease up pressure on the brake pedal just before accelerating. If you just hang on the brakes a tad, I've heard that moan.
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billccm wrote:

the front end of the engine where the rotating components like the AC are located. Could the AC be turned on? If it seems to be coming from the transmission/torque converter area the torque converter could make a noise like that, but more likely it's some component up front. Maybe the serpentine belt is frayed and ready to break.
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It's an old man's car, not surprising.
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