Lollipop's own notification sound

This is driving me CRAZY!
Ever since android 5.0.2 os came out I get a notification sound for all apps - separate to the one I assigned to them.
I can't find a way to turn it off because it has nothing to do with the app.
I can turn off the app notification, I can mute a discussion and while I may not get the app notification I will get this android os one.
I have went into settings, sounds and notification and made sure the settings were correct. I went into the app and turned off the notification and yet I still get this chime.
I was told I could swipe down from my home scree and find the lollipop setting but when I swipe down I just get the normal phone settings.
I have an HTC One.
Please help!!
Thanks

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Lollipop's own notification sound