Comment 14 for bug 1512002

I think the advice to disable the polkit-gnome-authentication-agent-1 to fix this is akin to advising someone to take the batteries out of their smoke alarms to stop the annoying noise.

One of the other links in there has a more plausible angle though, which is that it is to do with the message notification panel. I'm running with this disabled/removed to see if it is the likely culprit.