2001 Aurora whirring noise?

I have a 2001 Aurora with 99K mile. Lately I've been hearing a loud whirring sound that seems like it's coming from the bottom of the car when I get over 30 mph and doesn't go away until I slow down to a stop. The noise is there even when I take my foot off the gas. Anybody know what this could be. Could it have anything to do with the transmission or possibly tires? I would appreciate any ideas.

Thanks

Possible it is the trans, does it change with engine speed?

No, it seems to be the same sound at anything over 30 mph. The sound completely goes away when I slow down, say to stop for a light. The trans seems to be shifting ok though. The noise seems to be getting louder the last day or two.

This sounds allot like my car before the tranny rebuild....what rpm's are you at doing 45mph?
If you are around 3000rpm in overdrive the tranny is having issues.
My car would drive perfect and I was most likely driving it without overdrive for a long time never really noticed it, I was still getting 23 mpg . Now with it fixed I am getting 28/30mpg.

At the end before I had it rebuilt it would make a wirring sound while driving and it would go away at idle or standing still.

Marc