A problem has developed in my 1998 prelude, and it intrigues me.

A few days ago, I noticed an ambient high pitched humming, almost the same sound you might hear after listening to very loud music for several hours, then receeding to a quite place. I didn't think much of it, because these noises can typically come from the pavment you're driving on.

Later that day, I noticed the sound remained (regardless of what pavement I was on), and that it increased in amplilude and pitch at higher speeds. THe sounds seemed to "kick in" at 30mph and 60mph.

Seems to be somewhat of a harmonic thing. Any clues?