Double notification sound every 12 hours (Galaxy S7)

[bilzmale]

Every 12 hours my S7 gives a double length notification sound. Each time it is a few minutes later so it advances steadily over the days. Currently about 15:18 but tomorrow it will be at 3:20, then 15:22 etc (times estimated).

I have turned off notifications for all apps in Settings except SMS/MMS.

Any ideas as it's driving me mad.