Unidentified alert (vibration and beep) every 5 minutes, even with sounds set to Mute and DND enabled

Every 5 minutes exactly, my Galaxy S9 G960U1 running Android 10 vibrates and then gives me a flat-toned beep lasting about half a second. It's not associated with any notifications, toasts, or changes in status bar icons. It happens even with sounds set to **Mute**, and with **Do not disturb** and **Airplane mode** turned on! The only thing that stops the beep is **Mute all sounds**, but even then it still vibrates, and of course, muting all sounds isn't any sort of solution. The only saving grace is that if I have a headset connected, it only beeps through the headset. Another oddity is that I've checked through every alarm and notification sound available on my phone and it doesn't sound like any of them! I don't recall ever hearing this beep for anything else before. I've tried all of the following: - Clearing notifications - Restarting the phone - Disabling NFC - Disabling **Accessibility > Advanced Settings > Reminder alerts** (it was off to begin with, but I enabled and disabled it) - In-App Notifications, disabling all notifications for every app that allows it - Disabling _everything_ in Notifications settings except "Do not disturb" - Installing a notification history app - Disabling the Google app (suggested in a YouTube video about unknown notifications) - Clearing the cache partition Additional information: - I haven't installed any new apps or system updates or made any configuration changes recently (except the above-mentioned notification history app). In fact, this started happening while I was in bed, this morning. - I have **Don't auto-update apps** set in the Play store, and haven't updated anything in the last 3 days - The 5-minute interval is based on when the phone was last started, not according to the clock, e.g. I was getting the vibration and beep on the hour mark, then at :05, :10, :15, :20, etc., but after restarting the phone at 5:05 it was happening at 5:07, 5:12, 5:17, etc. I verified that the first beep always comes 2 minutes (_exactly_ 120 seconds) after the lock screen is displayed, and then exactly every 5 minutes, to the second, thereafter. I understand that a factory reset would very likely get rid of the problem, but I don't consider that a solution, that's giving up without a solution, and the total time cost would be more than I can afford right now, so I really hope someone knows what's going on.