OS X :: Constant Incoming Traffic From No Known Source?

System: uMB 2.4/4GB Ram, October 2008, running 10.6. In the past couple days, I've all-of-a-sudden noticed that I'm receiving incoming network traffic; sometimes it's 300-400kbps, sometimes it's almost 1Mbps. It'll occasionally stop for a period of a few seconds to a minute, then pick up again. Disconnecting the computer from the network will stop it (obv), but it picks right up immediately upon reconnecting.

No programs are running that would seem to cause this behavior. Nothing is being downloaded (I mostly do that on a separate machine anyways) and nothing is running when this begins that would lean me towards a problem. Switching from a wired connection to a wireless connection does stop the download completely, which lends me to think that this is most likely an attack on that IP, though I can't find a way to be sure or to even look further on that idea.