Thanks again, one and all for your hints, tips, advice and all purpose info.
I've just backed up the entire hard drive on both DVD and an external HDD.
Prior to that job I removed the side panel and applied right ear to the area around the hard drives.
The noise definitely came from that location (at the front of the case) and not from the PSU at the rear. Of course, with the certainty of night following day, there hasn't been a squeak (or a click) from the machine during the whole of the current session. However I think it would be a good idea to keep a new HDD in stock.
Much obliged! I shall now leave you in peace
Cheerio,
N o t a