Likely RFI, we have an issue at my local music venue. You can filter all you like, use filters on the power, DI the guitar, use hum eliminators - it's still there. We know it's radio borne, the guitar picks it up using the strings as an antenna and pumps it in with the signal. In our case it's an appliance that switches on and off randomly like a refrigerator, chiller or glass washer. We got a guy who is interested in Amateur Radio going to try and hunt out the culprit this week. Halogen light starters and Fluorescent starters can be a bitch for doing this, but that's easy to trace, turn the lights off, if it stops - it's one of them
Steve