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Hello and thanks for using Just Answer.I will try my best to help you with your question.
Do a quick check for me.
Take the lid off the air filter housing like you are going to change the air filter and listen for the noise. You may have a sticking IAC (Idle Air Control) valve. This valve allows air to engine the throttle body and into the engine when at idle and your foot is off the gas pedal. If the noise can be heard in the air filter box, change the IAC valve and the noise should go away.
The IAC causes a noise that will sound like a howl or a cow moo when at idle. It is basically air whistling through the IAC causing the noise because the valve inside the valve is partially open and sticking.
Wait and see.
Is the noise a constant howl or does it go up and down?
Where you able to hear it with the hood open when it happens?
Lets make sure before you replace any parts. I am only going off your description. But when the noise happens. Open the hood and listen in the air filter box or, you can unplug the IAC valve and see if the noise goes away. Below is what your IAC valve will look like and where it is located.