A bit more information, which has been reported to Norton:
Norton 360 makes a call to ipm.avcdn.net every 20 minutes or so, if it is able to reach that URL.
If ipm.avcdn.net is blocked, Norton 360 attempts to reach that URL every 30 to 40 seconds. Each call is accompanied by a flash of the cursor with the loading icon and a brief spike in cpu usage.
At some time after the PC is turned on, the attempts change from a couple of times per minute to a call every two or three seconds. Occasionally this occurs almost immediately after power on but most of the time it takes between 30 minutes and two hours to start.
The high calling frequency of this URL is accompanied by high cpu (~50%), a flashing cursor with the loading icon and, I believe, the occasional loss of focus. This behaviour is presumably caused due to looping/ waiting for the URL to respond.
As soon Norton 360 is opened to the desktop, the high cpu usage, flashing cursor with the loading icon and calls to ipm.avcdn.net stop (or probably occur at a low frequency – not tested).