CentOS 4.4, KDE, 512MB RAM.

I sometimes hear harddisc activity. Aside: This is probably an indication that I need to replace my harddisc but that's not my main concern.
I want to know what's causing the activity but I can't think of a way to find out. Given that I use KDE and only have 512 MB RAM, I suspect it's the OS swapping but I don't know for sure.
It happens after some use (I mean, I've logged in and I've started an app and then after a while it occurs), with a few services running and having Firefox 1.5 open, for example, but not doing any read/write myself.

Is there a way to monitor harddisc activity (like having it written to a log) with finding out what caused it?

Not really - not down to what application.
Centos will probably have the sysstat package - including iostat. Have a look at that.
Also try vmstat.

thx syg00, I'll have a look into that.