When I open System Monitor, the Network History shows that there are ~64 KiB/s being downloaded constantly. And it's not my doing! So I wanted to see which application is downloading and I installed nethogs.
But according to nethogs my system's download speed is less than 1 KB/sec in total!! What's going on? How can I make sure who's telling the truth? Personally I trust System Monitor as I can completely feel the lack of bandwidth. Then I must ask how can I find the process responsible for the download?
Here's how I ran nethogs:
$ sudo nethogs eth0
And there are only two network interfaces on my machine, eth0 and lo, none of which is downloading the 64K!