I commonly have my Air connected to a wireless network, and an ethernet network.
From time to time, I leave Activity Monitor open in the background. I often notice that there is a lot of network traffic, even when I'm doing things that aren't directly creating this traffic, i.e. the computer is idle, or I'm typing in a TextEdit document.
Commonly, the Data sent/sec is zero, but the Data received/sec often hovers around 20KB. I have Software Update set to automatically download updates, so I'm assuming it is creating some traffic.
However, I'd like to be sure. Is there any way to monitor which application is creating network traffic? Or, to monitor which interface (wlan or ethernet) it's using?