Static Discharge

Hi Folks,
Here's an interesting phenomenon I observed recently and I was
wondering if anybody else has seen anything like it.

About two weekends ago, I was doing a cross country in actual (IMC)
with one of my Instrument Rating students near Cheyenne, WY. We were
at 11,000ft for a while and the temperature was near freezing but
there was no precipitation in the area. All of a sudden, there was a
sound like a pebble hitting the windshield. I explained to my student
that it was either an isolated hail pebble or ice coming off the
propeller. About ten minutes later, I heard the same sound but this
time it sounded more like an electric discharge similar to the one you
hear when you touch the door knob after walking on the carpet. I
rechecked the fuses and the alternator but everything was working
fine. The sound was definitely coming from the outside. Then about
five minutes later, we heard the same sound. But this time, I was
trying to read the mag compass which is mounted up high on the
windshield. I'm pretty sure that I saw a flash of light this time near
the top center of the windshield where the Plexiglas meets the
aluminum . This made me wonder: When flying in IMC over an extended
geographic area (Cheyenne to West of Denver) is it possible for the
airplane to start discharging by arcing? Has anybody else seen
something similar to this? I should mention that the winds were not
especially fast that day (about 160 @ 30kts).