I certainly appreciate everyone's thoughts on what might be the cause of the snapping sound. It really does take a village to watch a movie at home!

Just to note, I get the snapping on UHD and standard Blu-Ray discs so this isn't just a 4K disc issue. However, the latest discs that I heard the snapping was Dunkirk UHD w/HDR that says on the box that it is DTS HD Master Audio and The Revenant UHD. The snapping also occurred when I played the standard Blu-Rays of Dunkirk and The Book of Eli.

As far as the volume of the snapping, it is proportional to the overall volume and occurs in at least the left, center and right speakers. I originally thought it was the just the center but that was because I sit closer to the center speaker than the left or right.

The 976's page on Outlaw's website does not indicate there is a firmware update unless I missed something.