Go to Settings > notifications + actions
Examine the list and look for an App that has "On: Sounds" as the only thing under the App title. Tap the title. In the field labeled "Notification sound" is the ringtone name... Tap that name and go to the list. The selected name is highlighted in your system default accent color. Tap the play icon to listen to that sound and confirm it is the one you are hearing.
Now after you found the App making that sound, you can do several things. If you never want that App to make a sound, scroll up the list of sounds to the top and tap "none." If you realize that is an App you want to hear from leave the selected sound on and tap the back button. Select "Show banner notifications" if you want a Banner on screen when the alert tone goes off and a notice left in Action Center. You can also select "vibrate" if you want a vibration when the alert first comes in. If you don't ever want alerts from the App turn the toggle switch to "Off."