What is this "Knocking" sound?

[Donna Walterscheid Endres]

Samsung Galaxy s5 - I am a new android user. Periodically I hear a sound coming from my phone that sounds like someone is knocking at the door. It's weird. Its not new email, new voicemail or new text. What could it be? Does it through the night too and my husband is not too happy with my new play toy.

 

[Golfdriver97]

Welcome to the forums. Have you checked against other apps like Facebook/Messenger?