on my dell inspiron 8100, when it's just sitting there idling, there is a regular blast of HD activity, just about every 5 seconds. just a "tick tick" ... pause ... "tick tick" etc etc.
Nautilus is stat()ing /etc/mtab. /etc's atime is being updated. The journal is flushing.
Try adding the "noatime" option to / in /etc/fstab. Either that, or use ext2 instead of ext3.
