......well, new to me anyway.
First of all, let me state that there has been excessive moisture in the air as of late, especially in the early mornings when I leave for work and it does not make this sound when the air is dry.

The last few mornings, it's been raining or foggy. I've noticed a howl that wasn't there before. I think my idler pulley bearing may be going but, every time I actually have the time to check it out, the air has dried out and it won't make the noise for me to isolate it.

 

Get up earlier and use a flashlight?

 

Does it do it at idle? Or while in motion? Or both?

 

Mostly at idle. It seems to fade away when I accelerate.