Book sound when in combat, how to fix?
 

https://www.youtube.com/watch?v=gvWoa8tH2m4

I disabled all addons and did this test today. Still makes the sound. So blizzard changed something in the UI or something?

Around 12secs into the video you'll hear the sound. Sounds like a rope and book or something.

Edit: Found out this is the darkmoon fair trinket fathoms

As you have already figured out, the darkmoon trinkets make this sound every time they shuffle in combat.
You can "mute" this sound by creating the following empty file:

World of Warcraft\Sound\doodad\g_booktraparm.ogg