Constant disk activity on NT Server sp6a

Hi, I have a new Compaq ML350 with 2 scsi hdd’s. It came with one and I installed the second myself. I then created a mirror-set of the two.
This machine is our Proxy server, and runs fine, web-browsing is quick and reading files from it is normal, however, both disks are constantly hammering themselves, even at 6:30 when most people have gone home for the night and there are maybe 2-3 users browsing.
Does anybody have any ideas why this is?
It doesn’t seem to be effecting performance, buti’d liike to know why as I have never seen it before and don’t know how to find out what is causing it.