On Thu, 21 Jan 2016 18:00:23 -0000 (UTC), Cursitor Doom
wrote:

On Thu, 21 Jan 2016 15:10:21 +0000, MJC wrote:

Could it be an electrical discharge? Try viewing in the dark (with
precautions against contact!)...

I like your thinking so I tried it. But no sign of any arcing at all.
Anyway it really does sound like more of a hissing than a rasping.
I have heard high frequency arcs that sound just like air escaping, at
least to my ears.
Eric