Drone noise at crusing speeds?
 

I've noticed a sort of ringing, hollow, metallic type droning noise at cruising speeds - I'd say speeds of around 50, 60 or 70.

It's not a loud noise, or sounds particularly worrying. Doesn't sound like wheel bearings. It sort of varies in pitch, slightly - sort of cyclically. Goes up and down. With the stereo on, you'd struggle to hear it, and with it off, it isn't a loud noise.

The car is: S70 2.5 10v auto (SE trim). Seems to do it, whether cruise control is engaged or not.

Anyone any ideas of what could be the reason? Like I say it doesn't sound worrying, I'm more curious really, and wondering whether it hints at some future trouble.

Lei Chan