I had same problem (2001 coupe V6), just that humming noise started at 40+mph. And it got louder at faster speeds.

I was able to determine the side (left or right) by going at around 40mph and driving on slight curves (such as exit ramps).
When I was turning slight left (more weight on right side of the wheels), the noise went away.
On the other hand, when I would turn slight right (more weight on the left side), the noise got louder.
Therefore, one of the left wheel bearings was bad.
I wasn't sure if it was front or the rear bearings...
Ended up taking to a shop, and they replaced the left rear wheel bearings.

Hope this can help with your diagnosis.