I switched my PC on last night to be greeted with a diddley-diddley-diddley-de. My, I thought, what a merry tune my PC is playing, never knew it could do that. My glee at hearing this tuneful melody was tempered seconds later by the sight of a blue screen and susequent boot failure 

So out came the screwdriver and off came the case cover. Out came components one at a time, sometimes followed by a diddley-diddley-diddley-de, sometimes not. As soon as it booted I headed off to BiosCentral to look up the code - not a diddley-diddley-diddley-de in sight, though I did spot a di-di-diddley-di or two  

So I continued with my quest of stripping the PC to its bare bones. Then I added one component at a time, booting after each and listening for the familiar diddley-diddley-diddley-de. After connecting one of my hard drives (backup), my little diddley-diddley-diddley-de reappeared. I disconnected the IDE cable, still a diddley-diddley-diddley-de. I disconnected the power cable, silence! Re-connected the power cable, diddley-diddley-diddley-de...

It seems on closer inspection one of the power pins on the hard drive is damaged. Easy enough to get a new (larger) hard drive. It could have been worse.

So the moral of this story is: if you hear your PC playing  diddley-diddley-diddley-de, don't just sit there humming the tune!