Glad you found and not a QC issue I was about to go run some music through mine for a while and then go and check to see if it would duplicate!
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Power Line Networking puts a ripple on the mains electricity, and is a nightmare for audio equipment. I have a tube amp that is totally unusable with it, and other gear that was also affected. I had to get rid of it, and run 30m of ethernet cable to replace it.
I also had a similar problem caused by the power supply for an electric reclining armchair, but that was easier to solve!
I am actually preparing to run a long cable through my walls to get rid of the PLC. Let’s see if it improves 
A UPS is your friend in these situations.
Indeed, I was thinking about this too. I think I will do both: remove the PLC plugs and clean my power using a UPS.