Why does my notification sound changes to "Pure Tone" on my Samsung Galaxy S5?

[CptnObvious]

I have a Verizon Samsung Galaxy S5 running Android 5.0 kernel 3.4.0. I use a stock sound (knock) for all notifications but at what seems like random intervals it will switch on its own to "Pure Tone"...a sound which isn't even an option for me to choose as a notification sound.

"Pure Tone" continues to play until the offending notification is acknowledged. It drives me crazy. I hate it. What is causing this to reset and how do I make it stop?

 

[ironass]

I have a Verizon Samsung Galaxy S5 running Android 5.0 kernel 3.4.0. I use a stock sound (knock) for all notifications but at what seems like random intervals it will switch on its own to "Pure Tone"...a sound which isn't even an option for me to choose as a notification sound.

"Pure Tone" continues to play until the offending notification is acknowledged. It drives me crazy. I hate it. What is causing this to reset and how do I make it stop?

In the first instance, you could try a cache partition wipe as per #1.3 of 41 Galaxy S5 models - Dummies Guide to clear out any detritus from previous firmwares.