Yes, hard disk recalibration makes a 'click' noise and then you hear it spooling up again. The IBM DeathStar was notorious for it - do you have one of those?
A new PSU will cost between 15 and 40 quid - a good one will be nice and quiet and pay for itself. Just take a look at the dust and cruft the old one has sucked up on the fan blades. You can always use it as your machine metamorphises over time.
If clicking persists, back up and see your HDD merchant in the morning.
BOFH