Worrying noise - PSU?

[None of the above]

This PC has started to make a loud and worrying click at random times. It sounds like the final noise the machine makes on powering down and can also be compared to a relay operating or a mains circuit breaker tripping.
Most components in the computer have been replaced or upgraded and the PSU, at four years old, is one of the few remaining original parts. Occasionally the PSU fan is quite noisy on boot up for a minute or two, so I suppose it would be prudent to replace the whole unit - it wouldn't break the bank. However, I'm anxious that I might be barking up the wrong tree.
Sorry, if the symptoms are somewhat vague but the noise sounds definitely 'mechanical'.

Any thoughts Gentlemen, please?

Thanks

N o t a