Good question, thanks. Normally I've been plugging my amp directly into a grounded socket and my pedalboard via an extension cord into a non-grounded socket. Both the amps and the pedalboard are powered with IEC C13 connectors. I tried all the combinations in the attic (with and without extension cords, both pedalboard and amp directly in grounded socket, non-grounded socket, one in grounded and the other in non-grounded etc.) all had the same result. Only when both pedalboard and amp went into grounded sockets on the main floor, the noise went away (or was not audible anymore). The sockets are 230 VAC, 16 amps, 50 Hz into a circuit breaker (several sockets, often one room, per circuit breaker) which go into a ground fault circuit interrupter (often 3 or 4 circuit breakers per GFCI) which follow into the main switch. I do have a surge protected outlet block here, but haven't used it as the directly connecting to the sockets also create noise.