A follow up.
The problem started the day I tried to use iTunes on a Windows box to set up my new iPhone 7+. iTunes wouldn't do a backup of the 6+ so I ended up doing the i7+ setup with iCloud backup.
Something on that windows box hammered the network, personally I think it is some process that got started that won't quit and starts back up when the machine is turned back on. So currently that box is off and will remain so until the start of a new cycle when I'll virginize it and sell it on Craigslist! Since its sole use was as a host for my iTunes library it isn't needed any more.
In fact I see no reason at all to use iTunes. Other than a few purchased movies and a few DVD rips, there isn't much in it anyway. And iTunes isn't needed, or at least I don't think it is. If it turns out I'm incorrect I'll get a used/refurbed Mac Mini to do it with.
For info, before I finally figured out what was doing it the data usage in those 3 days was 1.6 TB!