Late 2010 13" Air humming?
Do you notice it regardless of what app you are using? I have seen this kind of thing before with various audio drivers. 

If you can pin point it down to one location/area on the computer you might think about taking it in for diagnostics to an Apple Store or AASP. 

If it is software related, try keeping an eye on Activity Monitor when it is getting close to the time-frame of the sound and see if you notice any processes starting up that use a hight CPU% or apps that simply stop responding.