Why does my android 6 make a coin dropping sound every now an then.

My android 6 keeps making a coin dropping sound. Its every once and a while. And nothing pops up on the screen. Its like a real coin dropping except it is not a real coin. It is super anoying and I would like to turn it off. How do I do that.
If you can tell me how it will great.

That sounds like a notification sound from a Samsung. I actually use that sound, even on my Nexus. You're getting a notification of some kind, or it was set as some other kind of alert.