I just got this from Drew (lead stove designer):

"Hi Jason,

Yes we have sen this on a very small number of stoves. Basically the wire mesh that makes the dome on the top of the burner sometimes has a lose wire at the edge where it is sandwiched into the burner assembly. If you look at the top these are spot welded in several places but not around the whole perimeter. Well, in one of the spots around the perimeter where not welded a wire is vibrating at warm up and making that high pitch noise. There is no danger or anything wrong with the stove, just an annoying sound."

There is a possibility of replacement if it is too annoying. You can contact Cascade Designs directly here.

Hope this helps...