@Missy85 Hello, first try clearing the cache of the Whatsapp app.
Access Applications and search for Whatsapp. In Storage clear the cache. In Notifications / Notification categories, deactivate and then reactivate the checks of all categories. If the problem persists, apply reset preferences from Settings / Applications, the three points on the top right-Reset preferences. Comment us, greetings.
@Missy85: The sound you're hearing may be the result of background activity from a particular app. To check, next time you hear the notification sound, please head to Settings > Notifications > Recently sent, and check the top of the list. If you're able to identify the problem app, tap on it and adjust the notification settings accordingly.