We had some teen and twenty degree days here for about a week.

Coming back from across town (16 miles) after sitting over night I noticed a whirring noise. It sounded like it was coming from the front passenger side dash.

I've driven once since and the noise is there when I start off cold then it is only heard at around 35 mph though it will sustain itself down to 25 mph once started.

It happens in gear and in neutral as long as the car is moving.

I found that I can not notice any noise with the passenger side or drivers side windows down.
I did a drive by of some high concrete barriers thinking that would surely amplify the sound outside back into the windows but it did nothing.

The sound is very noticeable with the windows closed.

The closest sound I relate to it is the whirring of a vacuum cleaner though not near as loud. Bearings come to mind and I have been reading about the center drive shaft bearings.

I'm perplexed as to why I am not hearing any reflected sound from outside the car.
I'm also not seeing any tests to localize the problem to the center bearings except jacking the car and running it up in the air. Otherwise I'm not seeing inspection tests such as a tolerance in degree of movement.

Are there other things I should look into?

I have checked the differential fluid and it is at the proper level.