After a year of two of virtual silence, my HD has started making a noise. You can't really hear it if you're watching something. It's 'normal' HD reading writing.
I was wondering if the HD was dying - we do record a lot and the Topfiled is on from 9am to about midnight with the disc on.
Ta

Just a thought - could it be that since you obviously use it quite intensely, recording and erasing, the disk has become fragmented like a PC hard disk in need of some housekeeping?

Fragmented disks certainly sound 'busier'.

Perhaps someone who knows more about how a Toppy uses its hard disk could comment before you go re-formatting it though.

You might find that changing the disk's AAM (acoustic setting) will make it less noisy (don't worry about the tiny change in "performance" this might make -- it's irrelevant for a PVR). FireBird's HDDInfo is a TAP that will let you do this on the Toppy, otherwise you can remove the disk, plug it into a PC and use any suitable AAM tool.

HDDInfo seems to have done the trick. I set it to quiet and it seems back to being near inaudible again.
Thanks R2D2

Since the disk is constantly in use, I'm surprised it doesn't defrag itself every so often.

The cluster size is more than 1Mb, so there's absolutely no point. (On a PC, your disk is likely suffer when fragmented since it's likely to have a cluster size of just 4Kb.)