Cannot stop random notification sound on Samsung S8+.

I have gone to notification apps and only have three apps that I allow notifications for. They are not incoming with any info). Yet I keep hearing the notification sound over and over again, every 30 seconds or so. No one at my carrier can figure it out. If I go to settings, Notifications, and go to volume, I can turn the notification volume off, but then i will not be notified when a message comes in, I will only be able to see it on the home page if I am near the phone. Any ideas?Where do I go to find out where the notification is coming from (if not from the apps), so I can stop the random sound every 30 seconds or so?

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