Where is this sound coming from?

[AC Question]

I have chosen custom notification sounds for most of my apps: e-mail, text messages, etc. Lately, I would get a notification and hear the sound I designated for that particular app or action, which is what I expect, but immediately afterward a brief chime plays similar to the sound made when plugging your phone in to charge. I am using a Samsung Galaxy S5 with Marshmallow 6.0.1. Where is this noise coming from? I've tried a notification history app, but that does little to nothing to tell me where it is or how to disable it.