NetHogs is a small ‘net top’ tool. Instead of breaking the traffic down per protocol or per subnet, like most such tools do, it groups bandwidth by process and does not rely on a special kernel module to be loaded. So if there’s suddenly a lot of network traffic, you can fire up NetHogs and immediately see which PID is causing this, and if it’s some kind of spinning process, kill it.
NetHogs is default available in Ubuntu repository, use Software Center to install it. Or:
sudo apt-get install nethogs
NetHogs needs root privilege and defaults to eth0:
sudo nethogs eth0
I’m using pppoe connection, so:
sudo nethogs ppp0