2015 Terrain, odd engine howl?

Hello there, I recently bought a 2015 Terrain and have noticed an oddity. It didn't do it on the test drive, nor on the way home. But the next morning on the freeway to work, the engine tended to "howl" at times, only after the engine was warmed up, and usually at a very slight accelerator position. Let off the accelerator and the sound stops, give it more accelerator and it also stops. Won't do it with a cold engine. The sound is like that of an empty bottle whistling when air is blown over the top.

I'm thinking it's literally something in the intake air stream (or exhaust?) creating the noise when the flow rate, pressure, and temperature are just right.

I googled this and found nothing, so thought I'd ask here. Any ideas?