Clicking from under the bonnet

Basicly I started the car this morning after a week away in Germany and when I turned on the rear window heater there was an audible clicking from under the bonnet from outside the car. As soon as I turned off the window heater it stopped and when I turned it back on it carried on for about 10 seconds and then stopped. I opened the bonnet to have a look but there did not seenm to be anything out of place. I thought nothing of it until this afternoon when I parked up after a 'spirited' drive and left the engine running while I grabbed some stuff out of the boot to let the turbo cool and spin down and as soon as I turned off the engine I heard the same sort of clicking from the engine bay for about 5 seconds.

Any ideas as to what it could be? I thought alternator until I switched off the engine and heard clicking.