Based on this thread, I contacted Nikon Support UK again about this noise.
I first queried it with them a couple of months ago and was advised to switch to Single Autofocus and the noise would stop.
This of course didn't happen so initially they told me to return my 8800 to trace a possible fault. Later they revised this and suggested the noise was normal.
This time I tried to pin them down and asked if they knew specifically what was actually making this noise?
They replied that it was "the internal mechanism of the camera".
In other words they are as baffled as we are.
One would have thought that after the countless queries they must have received about this noise, they would look into it in more detail and come up with some answers.