whirring noise at 2000-2300 rpm's or so

Just started a few days ago. Especially now that it's well below freezing in Denver.

At around 2000 to 2300 rpms there's this whirring noise that comes and goes. if i accel harder, it goes past and is fine. the noise is intermittent.

it also happened just at startup today when it was 5F outside for long enough for me to open the hood.

it sounds as if it's coming from under the airbox area - not coming from the belts.

I've tried to rev the engine while at a stop to replicate the sounds, but i cant really do that.

any thoughts?

 

It’s directly related to engine rpm. Will happen when car starts this morning in the very cold. Then go away. Happens intermittently while driving. Speed doesn’t matter.