Anyone having notification tone issues?

I haven't added or changed any apps recently. But today, my phone has started sounding the default notification tone about every 3 minutes or so, any time I have a notification icon on my phone. Sometimes even when there's no icon displayed. The only way I've been able to get it to stop is to turn off ALL notifications in ALL apps.

Has anyone ever seen anything like this before, or have any idea what I might look at?

Thanks.

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