On my '05 there are two speaker-like devices in that area. One if for the Tag Axle "raised" alarm, and the other is the Turn Signal "on" chime.
I think there is also a Low air pressure buzzer in that vicinity. I suppose any of these could be malfunctioning or getting feedback resulting in noise.
Do you have the removable dash access panel above the steering wheel? If so, pop it off and see if you can narrow down the source of your hum.
Good luck.