Mystery sound?

[Pentalobe]

I've got a Samsung Galaxy Nexus running 4.2.1. My GN has made some mystery sound once or twice recently and puzzled me greatly. I looked and looked and I had no clue what triggered them! I checked and I had no notification from Facebook, email, sms, etc. In fact I hate notification sound so I had already turned all of them off. I just leave the pulsing notification light on.

Any advice or suggestion where to look will be much appreciated.

 

[whitespace]

What does it sound like?! I realise that describing the sound could be tricky; but give it a go!

Is it a short notification-like sound? Or longer ringtone-like? An alarm?! Battery low (does the battery-low indicator play audio?)? Are you able to find this sound bite on your device (using a File Manager of some kind)?

Any other notification type apps... WhatsApp? etc.