What it could be is simply ARP traffic. The newer modems such as the 7500 and the newer ActionTecs constantly scan the network via ARP Traffic to find out what hosts have what IPs and then populate the network list. This as a result also flashes the Internet light for a reason unknown to me (there's nothing to find!) but if you shut the Network Scanning off in the modem, I can guarantee the excess activity will stop.
You may want to run Wireshark on an attached device to see what the extra traffic is with no other connected devices.
I know in the Westell modems, when you visit the "My Network" section of the modem there is a button on the right hand side of the page to disable Network Scanning. I don't have an accurate location of where this may be in the ActionTec, but if it is present it should be found in about the same location.
No luck YET in finding anything in network stats that would turn off this network scanning bulls**t. This new modem appears to work better - but do NOT like this constant flickering of my nic card's light and the associated traffic - however benign.