I have the same problem. Found this thread while googling for a solution.
My phone is Note20 Ultra 5G. Amazing that through all these years (from the thread chain) the same problem is there with different triggers and solutions. I used to have it the last few months I was using my previous phone, Note9, then I switched to my current phone and it was gone (I transferred all my data and settings using smart switch app). Only yesterday (5-Jan-2021) I started getting the same annoying notification sound!
Some facts and trouble shooting efforts:
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1- My phone (N20 U5G) recently got the Android-11 (one UI 3) update (last few days in Dec-2020)
2- I have my notification reminder off
3- I tried switching off NFC and bluetooth
4- I tried cache reset
5- I tried switching off ALL apps notifications
6- It is NOT "technically" a notification, but it is fired through the "media" sound category. It responds to volume up/down of the "media" sounds in my volume controls
7- It sounds even when I am on a call (!!!) in my bluetooth earphone
8- When it sounds, there is absolutely nothing showing in the notification bar
9- The sound it plays does not exist on the Ringtones list, Notification sounds list, and is not a sound file in my phone's folders (as far as I have searched) !!!
10- It is seems completely random to me, as I could not associate it with any action, app, motion or time!
11- It went away when I rebooted my phone into safe mode, for as far as I have kept it under monitor
Hence I started deleting some of the applications which I use rarely or can be deleted (stickers and stuff). It is still sounding so far. I am not sure if I am on the right path.
Appreciate any suggestions. It is really annoying.