98 making a loud humming noise while accelerating

I have a 98 with 120,xxx miles. Just recently it has begun having a loud high pitched humming noise while I accelerate. It drives great and doesn't seem to be affecting the performance, but it sounds pretty bad. Any suggestions?

Thanks.

Maintenance is not something to wait on. I'd check out what's going on where so as not to end up stranded over something silly just overlooked or neglected.