Hi Loui77,
Thanks for using Apple Support Communities. If you go to Settings > Sounds & Haptics, then look through the ringtones and text tones, are you able to find one that matches what you hear? Does this coincide with anything in particular such as a message from a particular contact, an email in a certain account, etc.?
If you haven't already, check your app settings using the guidance in Change notification settings on iPhone - Apple Support.
"Change notification settings
Most notification settings can be customized for each app. You can turn app notifications on or off, have notifications play a sound, choose how and where you want app notifications to appear when your device is unlocked, and more.
- Go to Settings
> Notifications. - Choose how you want notifications displayed on the Lock Screen:
- View just the number of notifications: Tap Count.
- View the notifications grouped into stacks by app: Tap Stack.
- View the notifications in a list: Tap List.
When notifications arrive, you can change the layout by pinching the notifications on the Lock Screen.
3. To schedule a notification summary, tap Scheduled Summary, then turn on Scheduled Summary. (See schedule a notification summary.)
4. To choose when you want notification previews to appear, tap Show Previews, select an option—Always, When Unlocked, or Never—then tap at the top left.
Previews can include things like text (from Messages and Mail) and invitation details (from Calendar). You can override this setting for individual apps.
5. Tap an app below Notification Style, then turn Allow Notifications on or off.
If you turn on Allow Notifications, choose when you want the notifications delivered—immediately or in the scheduled notification summary—and turn Time Sensitive Notifications on or off.
For many apps, you can also set a notification banner style and turn sounds and badges on or off.
6. Tap Notification Grouping, then choose how you want the notifications grouped:
- Automatic: The notifications from the app are grouped according to organizing criteria within the app, such as by topic or thread.
- By App: All the notifications from the app are grouped together.
- Off: Turn off grouping."
If the same continues and does not coincide with any app settings, it may be best to reach out to Apple Support for a closer look- Get Support.
Kind regards.