Need help on diagnosing this noise on my 98 DX.

When driving normally, I get a metallic humming noise at around 40-45mph. The noise goes when I press down on the gas and the speed goes up, but as soon as I hit around 45mph, under 50mph, the noise comes back. Any suggestions on what it could be?

I'm using the stock steelies and I haven't really checked the condition of the rotors or brake pads since I bought it, but brake still feels ok. Will change rotors and pads pretty soon. But they don't make noise when I use the brakes, etc. So i'm thinking it might be something else.

Just background info... the car has 88K+ miles, stock suspension(still good, rebound and stuff is good), stock steelies with a/s tires, stock clutch, haven't changed timing belt and stuff but generally in good running condition. Most of the general maintenance on it has been done since I bought it, but still a work in progress.

Could it be the wheel bearings? Belts? Pads/rotors? It loud enough that I can hear it over the radio and seems to be louder with the A/C on at setting 2 at the least. i can still hear it with the A/C off so that's why I'm stumped. I thought it was a belt that was slipping or something.

any help would be appreciated.