Thanks again chaps. I ran the diagnostic procedure - no result. The tickety tock is only there when the ATC is on AND the engine running. But...it is definitely the balance valve because I managed to make it stop by changing the balance. I guess I normally have the temp set the same on both sides of the car. But if I adjust it so that there is more than 2 degrees difference, the tickety tock stops. And as I adjust either temp setting, you can hear the valve operating with the same noise. Don't know why it keeps operating when the temps are the same. Interesting...

So it looks like she will have to sit in the cold while I enjoy optimum temperature! And at least I can listen to the locusts in peace.