Air Line Arm Owners- Compressor Electrical Noise

As mentioned in another thread, I finally got the Air Line arm installed on the XL table, on the finite stand, etc. I installed the arm compressor in a separate 'room' (in reality a large walk in closet off the listening room) and powered the compressor from a 'regular' circuit outlet- the system is otherwise hooked up to its own dedicated lines, per standards discussed many times here.
I have been experiencing a periodic electrical 'zap' (sounds like a big ol' scratch on the record) through the system when the gain is up. Since the room lights dimmed when the compressor kicked on, I figured- why fool with the standard circuit to feed the compressor? I had the electrician come back and set up a separate line just for the compressor- having nothing to do with the system wiring or subpanel. (He had already pulled extra wire through the conduit up into the room, so it was no big deal to do this- he installed a high quality receptacle to this extra line, and terminated it at a different subpanel than that which runs the audio system).

Anyway, still that nasty zap. Never out of options, I decided to plug the compressor into a 240v-120v stepdown transformer I use to power the HT system in the same room. (The HT system is completely separate from the analog hi-fi system we are concerned with here).
So far, so good- the compressor kicks on, but no zap.

Still monitoring the situation- wondering whether other Air Line owners have experienced similar problems (which may be reassuring if this is the cause and I licked it) or I am still chasing electrical gremlins. TIA.