iPhone XR keeps making notification sound when there is no notification

Greetings, bryson31.

Welcome to the Apple Support Communities. We can understand why you’d want to resolve the issue with this unknown notification sound. 

Check to see if you have any audible alerts enabled.

“Change alert styles

1. Go to Settings and tap Notifications.
2. Select an app under Notification Style. 
3. Under Alerts, choose the alert style that you want. If you turn on Allow Notifications, choose when you want the notifications delivered — immediately or in the scheduled notification summary.

With iOS 15 and iPadOS 15, you can turn on Critical Alerts so notifications come through even if your iPhone is muted or you have a Focus set up.” Use notifications on your iPhone, iPad, and iPod touch 

If not, be sure to Restart your iPhone and ensure that your iOS is up-to-date: Update your iPhone, iPad, or iPod touch 

We hope this helps! Thanks for being a part of the community.

Sincerely.