Unknown notification sound with out banner

I'm using Samsung Note 8, I'm getting unknown notification sound from my phone but no visible notification, popups or banner from any of the applications. This sound is not even in my notification sound list. Very annoying sound like Tu Tu Tu....
I disabled all notification sounds, put in Do not Disturb mode, still sound comes. There is no sound when I switch off wifi or data. I installed several system notification log apps... in that also not showing.
I disabled notification reminder, disabled NFC. This is not related to NFC this sounds even it is alone in bed or table.
I don't remember when it started.. may be 6 to 7 months... still it is there.

Please help...