..... re-connected the power cable this morning.... after 7hrs of cooling.... and the clicking started straight away.... without firing the pc up.
All external peripherals where disconnected.
Just to make sure that it wasn't an electrical short.... I turned all the lights off, and had a look at it in the dark.... nothing. Although a light does come on the motherboard, which I didn't expect.
The noise only disappears when the system is fired-up, only to return once powered down.