Ok, I have a Nexus 7 2013 running Android 4.3.1 and Cyanogen 10.2.1-flo The other day I started getting this noise randomly and when I unlocked the device: (At about 7 seconds into the video. No it's not my video or my tablet, just an example of the sound.) I have no idea what the sound is being made by. And I have not installed much lately. Facebook messenger (Uninstalled again) seemed the likely culprit, but the sound is still there and i'm baffled. Certain things point to "visual Voicemail" but this is a tablet, not a phone and has no voicemail apps or phone options on it at all. ANYTHING would be helpful. Thanks!