The sound is comming from the case, appeared to be the front of it. When io press the power button is it does a run of clicks on top of each other, a 'flutter of them'.
I did some investigation! As soon as I put the case on its side it had stopped doing it! I kept on turning it on and off but I couldn't find where abouts the click was comming from, probably more the front side where the hard drives are and power button is.
When I first installed my PSU I kept some of the wires tied together. The ones that were not used at all, with a tie, out of the way of the motherboard, and below the hard drive. The plastic covered ones with a wire running down the middle that keeps cables wrapped up in the box new. I removed this, and let the cables hand free. There is a big space there so I think they should be ok there for now. When I turned it on, the flutter noise seemed to have disappeared. I still the intermittent clicks though.
I don't understand why undoing a tie around left over powercables would make a difference. I guess they could be acting like a coil, and causing something to move. Who knows. The wires are still underneath the harddrive