What is the notification sound on my HTC 530?

[AC Question]

Thanks in advance. I'm technologically somewhat useless, but nobody else in work can figure this out. I was listening to YouTube last night when my phone randomly started making a noise and it's done it intermittently ever since. I tried getting a recording but the sound is too low - you need earphones in to properly hear it. The best way I can describe it is like some sort of notification. Almost like a Skype call incoming, a couple of short tones and a longer one. But everything is closed, I've used the task killer and have no apps open. It does it intermittently too, even when I'm playing a game, browsing the web, whatever the case is. I tried switching it off and on again (I've seen The IT crowd, I know how it works!) and even at the start-up screen it started doing it. I'm suspecting a software fault but thought I'd see if anyone could shed any light?

 

[belodion]

Welcome. Try first a Cache Partition wipe.
If no go, try booting into Safe Mode to test whether a third-party app is causing trouble.
If still no go, get back to us right here.