Well, I finally got lucky and now the noise is completely eradicated! This fence had three separate "gates" consisting of a "gate handle" in each of the three wires. While the handle contacts with the wires looked good, there must have been internal corrosion involving the spring mechanism. I replaced the gate handles ($2 each) and the problem is now gone. Funny you should mention it, but he actually is using a piece of rebar for the ground rod! Fixed that early on, as the ground clamp was a bit larger than the rod and it was loose on the rod. I really don't care how efficient his fence is as long as it's not arcing, so I left the rebar in place and shimmed the clamp with a strip of copper sheet. Luckily, we have a big farm supply store about 25 miles away and they carry just about everything in the way of electric fence supplies.