Hi Andy, Once I got my S back on the road I too noticed a tapping sound that started from cold and then went away once the car warmed up. It had a distinct sound similar to dropping a marble on glass! I seemed to come from the top of the engine, but it was difficult to pinpoint exactly where. It was not a clatter (tappets), it was not the injectors, and although it was cyclic it wasn't continuous, it would come and go. Does this match your symptoms?
Anyway, as I didn't want it to go untreated I decided to take off the belt covers first and just check that everything on the front was OK. I started the engine and watched...what was actually happening was the cambelt was vibrating as it left the cam pulley and hitting the cam cover and the water pump guard fence. This was causing the tapping sound, basically the belt needed re-tensioning as it had stretched slightly. However, as the engine warmed up the tapping became less - as the alloy block expanded and took up the slack in the belt.
So, did a re-tension (purely by feel) and the sound has not returned...but I may have tensioned it just a bit too much as the belt now whirrs a bit like a washing machine.
If you think it might be this listen carefully to where the sound might be coming from, use the old screwdriver against the ear technique, or if you have a stethoscope so much the better. Obviously be very careful around the rotating bits!
All the best, Rhett.