More Electrical System Madness

So, after getting the new subpanel, dedicated lines, hubbell receptacles, fiddling with grounding arrangements, etc., system is dead quiet (remember, it is a horn-based system, about 104db efficiency, so I'm hearing alot of stuff I wouldn't have heard on my old electrostatic set up).

What is disturbing me is a nasty 'snap' that goes thru the system periodically, sounds like a ground disconnect/connect at gain (or a monster scratch on a record at full volume, but that ain't it). At first, I thought that the Air Line arm compressor was surging when it kicked on, even though that is powered thru a separate wiring set up than the hi-fi. (It was plugged into the regular circuit in the room, not the dedicated circuits). So, I had another dedicated line, etc. hooked up, terminating at a different subpanel, to run the compressor. Now when the compressor kicks on, the room lights no longer dim, and no compressor power 'surge' is heard. But, there is still this periodic electrical 'snap ' through the system, and I don't think it coincides with the compressor activity. I rechecked all my equipment grounds, tight and true, don't think it is a bad tube (is it?) and otherwise am clueless as to the origin of the problem. I thought the dedicated lines would eliminate any interference from appliances in the house. Assuming that's true, then the problem either originates from the power company, or the dedicated electrical system in my house, or one or another pieces of equipment.
You hear this snap when a record is playing, and to be sure, if you cue the arm up, but keep the gain up, it will periodically snap thru the system. I haven't timed it to see if it occurs at precise intervals yet.
Any ideas on cause, or approach to solving this?