Drive Activity on Idle Laptops

I have two similarly fitted out Thinkpads of the same vintage running
Win2000. On one of them, when there is no computing going on, the machine is
truly idle, at least to outward appearances. There is only very infrequent
random blinking of the hard drive activity LED and corresponding drive
noise. On the second computer however, there is constant fixed interval LED
blinking and drive clicking - I'd say about once per second - when the
machine is idle. I don't think it's the installed applications, and in fact
I removed Goback and unplugged the internet connection as potential
culprits, but neither made any difference.

I'm wondering if this is normal, and if it isn't what might be causing it,
and whether it is benign.