HDD showing activity with no users

I have a RH 7.3 server that is continuously showing a lot of HD activity
even when no users are connected. I assumed after a while of inactivity
everyhting would end up in cache.

With about 130 processes runing, is there anyway to find out which process
is accessing the HD? top provides CPU and mem usage but can't find anything
that relates to the HD.

Thanks
Daniel.