That would be the high frequency power supply that steps up the voltage for the electroluminescent display. Depending on the design the frequency may be in the audible range. The modulation is probably due to varying load as the display changes.
This is similar to the squeal from the scan frequency in old TVs. Some can hear it, some can't. Once you hear it you can't unhear it.
However, this seems overly loud and low in frequency to be so easily recorded.
