I heard a story from a guy who worked maintenance for Comcast in the Seattle area about a bizarre fix for this kind of thing. They had a neighborhood that was doing offline everyday at the same time, like from 8-10 pm, everyday like clockwork and they couldn’t get a fix on the cause. So they finally got a couple nightshift guys in bucket trucks to sit in the neighborhood and, using online tools to monitor the node, watch for the noise to kick in. One guy was sitting in front of a house and right about 8, he notices a person in the house sit down in front of a window and turn on a light. Instantly he starts seeing noise in the node. On a hunch he went and knocked on the door, explained what was happening and asked the customer if they could indulge him and just go turn that light off ta few minutes. They do, and the nose goes away. The customer let him in to investigate and it turned out it was an antique lamp with a fraying old fabric wrapped power cord, plugged in to the same outlet that the modem was plugged into. So somehow this lamp was feeding voltage or low frequency noise into the outlet and the modem was picking it up through its power cord and feeding it back into the system. The customer said it was his nightly before bed routine to sit in that chair under that lamp and read a book. They had to ask the customer to please not use that lamp, or get it repaired, or move the plug for the modem to another outlet He said it was one of the more bizarre coincidences that they were able to just witness the cause. If they hadn’t seen it, it might have taken a few days of nightly monitoring to chase the nose down to that one particular house in the 2 hours it was happening everynight.