Is it perhaps an NFC connection being made? That makes a sound that sounds a bit like trickling water/chimes and won't indicate anything on the screen.
Perfect. It was my bus card in Lumia 950 wallet. Thank you so much!This is the sound I am hearing: www(dot)youtube(dot)com/watch?v=o6EsxMLabgs (new user here)
This has been annoying me for so long, I snapped today and found this thread. I turned off NFC in the settings and I will see if I hear it again.