I have champions installed and an osvermis became a champion and then spammed the digging sound. I think it has to do with the effects it had. Here is a screenshot

This osvermis was definitely the culprit because after I killed him the sounds stopped

It seems to happen regardless of the effect

In my experience this is specifically with the Molten affix; if you blacklist molten for osvermis you should find this "solved."

Fixed in the latest 1.2.4 version.

Download Link:
https://www.curseforge.com/minecraft/mc-mods/fishs-undead-rising/files/3282949