I'm thinking this must be driver-related as it has only bugged me now that I'm using XP (on this 900mhz, 512mb T22). Somehow some inner activity is feeding into the audio, noticeable when using the trackpad/pointer (old style without touchpad) but also when programs are opening and so on. If I plug a usb mouse in I can here very quiet whining, almost undetectable, but I've never had this with another machine.

From googling I can see that possibly similar sounds have been heard by many others, though no one seems to have got to the bottom of this, with various things being a partial solution. The power plan used, and whether I'm uising battery or mains doesn't make any difference. I'm fairly sure that Windows 2000 and various Linux distributions did not make the same amount of noise if any.

As other remarks about this were made when the machine was new or fairly new I'm not putting it down to age.

When XP is installed only one driver needs to be added, the power management one, and I wonder if there's a connection. With W2K and W98 eight or more drivers need to be added manually.

When casually reading up on the T series I may have seen a post that had a solution for this but it may have been Linux-based, possibly in alsamixer. But nothing in the Windows equivalent seems to show any channel being on or too loud that shouldn't be.

I will learn to grin and bear I imagine, and as of today or tomorrow when my class 10 SD card arrives for a 'Frankendrive' experiment XP will likely be for occasional use anyway, but if anyone has any useful comments that'd be great.

I'm a bit puzzled why I can hear a noise when the speakers are muted though - what else can a noise come from of this kind?

The hard disk, the fan, the CD-drive.
More difficult to trace, the inverters.
See this picture of the mobo top: http://www.rkawakami.net/ibm_t2x/mobo_top_72_legend.jpg
- big rectangular above left of the CPU (at location A1/A2, but it's missing on this picture)
- square 'blob' at C5/C6 and G6
And on the bottom: http://www.rkawakami.net/ibm_t2x/mobo_b ... legend.jpg
- square 'blob' at D9 and E8
These last two have a nasty habit of sometimes falling off due to bad solder joints.
If you have the mobo out, just try to 'lift' them off putting a fingernail under it.
If they do, scrape the metal contacts and the matching solder points on the mobo and solder them back on (either way is fine).
Don't mix them up, they have different values.

Great, thanks. Haven't soldered for a long time though...

I was going to attend to the fan anyway but wanted to ask - should both a thermal pad and thermal paste be used, or just the pad as currently present?

Depends on your fan.
The T20/T21 fan (has drilled mounting holes) comes with a thermal pad, no paste required.
A real T22 fan (has spring-loaded mounting holes) requires only thermal paste.