All very good questions. Is the traffic all incoming, outgoing, or a mix? I always notice that there is nearly constant activity indicated on my cable modem, but it doesn't translate over to my wireless router nor my LAN home network. There is a little app, Little Snitch , that will alert you to nefarious activity (actually, it will alert you to all activity unless you tweak its settings). Another thing to check is the built-in firewall settings in OSX.

System Preferences>Security & Privacy>Firewall

I have mine fairly well locked down with Stealth on and it doesn't affect my ability to connect to anything.