Need advice with clicking noise from my tube amp

I have a Don Sachs tube preamp and amp.  Recently, I notice a consistent "clicking" noise, about 2 seconds apart, from both of my speakers while listening to my system.  I don't recall the clicking noise changing at different volume levels. I swapped the Sachs amp for two other amps, both solid state, and there was no noise, so I concluded the issue was with the tube amp.  

I took the amp to a topnotch tube tech who listened to it in his system for several hours and did not hear any noise, clearly not the clicking I heard.  The tubes were checked and the biases is spot on for all four KT88's.

I asked Don about the possible cause and he is stumped if it can't be replicated with the amp in a different system.  One possible thought was that the amp is picking up some RFI from some source, but I can't imagine what that source might be.  I don't want to have to build a Faraday cage around my rack, as if I had the KSA's  to do that.  

Any thoughts from you folks would be very much appreciated.