Via a network traffic monitoring tool I see that my laptop is generating lots of outgoing (EDIT : incoming !!) network traffic.
Although no download program is running or any other program of which I know that could be generating this much traffic.
Something strange is going on and I need to know how I can find out which program( s ) are generating network traffic.

Someone can point me out where to search in my system ? Which command to use ?

Ethereal (or is it wireshark now?) will give you a lot of info on the packets. I doubt it can tell you the application, but it will tell you source and destination ports, packet contents, etc.

This happens everytime I boot up my Fedora 9.
It just keeps on getting megabytes of incoming traffic. To give an example : in 15 minutes my system got more then 500MB of incoming traffic.
It's not outgoing, but incoming !! My bad.
I'm wondering where my system puts all this megabytes, all this incoming data ?!

What I do to stop it is : I unplug the WAN-cable of my router. I see on my monitoring program that network traffic stops. Logical, cause I have no longer an active internet connection.
I plug the cable back in, and no longer there is network traffic going into my Fedora-system.

Just the usual I generate by browsing with Firefox and so...

Is it abnormal that yum should generate this much traffic for checking updates ??? I personally don't think it's yum.

Wireshark is indeed an option, thanks. But I still would like to know the program/process/deamon.

Have you got automatic updates switched on or similar?

Well, at startup my system will automatically check for updates, yes.
But still : 500MB in 15 minutes ?!

And when I manually type 'yum update' in terminal, it tells me there are no updates.

Sounds like it's checking and installing without interaction.

Installing what ?

I found the program that created the network traffic : mail notification.

I have a program that notifies me when I recieve mail on my online mail-account, without having to open Evolution.

By killing this proces, the network traffic stopped.

So it seems that this really isn't a good program !