> It's behind a UPS and a good surge protector.

To be clear, neither of these things are intended to address EMI/RFI. Most consumer-grade UPSes directly pass the AC power through when not on battery, so any noise on the lines will also pass through pretty much untouched.

Surge suppressors are just MOVs (metal oxide varistors) and a circuit breaker. If the line voltage rises too high, the MOVs try to shunt the voltage. But if the EMI's peak voltage is below the MOV's trigger threshold, it will do nothing and the EMI will pass straight through.

My understanding is it's the piston and gas, undergoing rapid decompressor.