Can't figure out notification sound

I just got the Samsung Galaxy S8 Plus, but previously had this issue on my Samsung S6 too. I keep getting a notification sound randomly without any apps being open, no window pops up with any text telling me what the notification is for.

I turned off most of my app notification sounds in the app settings. I've also tried downloading Notification History to see what pops up, but it doesn't detect the noise.

Anyone else have this problem or have any recommendations?