So I'm getting a noise...

[thaduke2003]

I'm getting a noise that is most noticable between 45-55 MPH, regardless of gear. It is a tuning-fork-type resonance- I can't hear it above 55 MPH, but that could be wind noise.
Any ideas? I'm thinking it may just be an aero noise, but it sounds as though metal may be involved. Thanks! Mark W.