Thanks very much for your input on this Jocko!
I think I now have a good understanding of what's going on.
1. Googling around, other cars/makes have similar problems. It's fairly rare - although Hyundai did a recall some years back
2. I used to have a Civic that
very occasionally did a one-off whirr - I just assumed it was some random actuator and didn't investigate. The Civic's adjuster was audible when used.
3. The Wheatstone Bridge circuit is inherently sensitive - traditionally it was used to measure resistance very precisely. I would
expect it to be quite jittery in response to various minor circuit problems
4. Lots of headlight adjustment motors on various cars may be operating
unnoticed - Because their mechanicals are naturally quiet
- Because they are further away from the driver
- Because the systems are only powered up when the lights are on.