You could go to Settings > Notifications & you should see what the Notification is from & you SHOULD be able to Toggle off the notifications from that source.
Sometimes you can get non specific System Notifications as well, but I haven't bothered to investigate it further, as it's not very frequent.
I don't remember if your device is supported by Good Lock's "Nice Catch" app, but that could also give you more information.
You could also Boot into Safe Mode & see if you continue to get the beep. If you don't, then it's coming from a 3rd party app which you installed.