Nokia charger

I've just noticed my Nokia 6630's charger is making a very quiet, but very regular ticking noise while the handset is charging. This got me thinking, does it have some sort of mechanical timer to turn off the current after a pre-set time? I'd have thought that was rather unlikely as the charger (model ACP-12X) is very light, but I can't think why else it'd be ticking like a clock