Fast idle to normal speed control

I started the engine from cold and raised the bonnet to hear if there were any unusual noises. The engine was in fast idle mode around 1100 rpm. I heard a sound from the right of the engine near the airbox like a spark plug spark 'click' noise with the engine dropping to normal rpm. The question for the technical-minded is what controls the rpm from fast idle to normal and what makes the 'click' sound? Engine is 1.2 tsi.