I had a patient with rather audible and palpable snap with neck rotation. She was concerned she had loose joints in neck. It turned out to be coming from a snapping Splenius cervicis muscle.
The splenius cervicis in the neck extends to T4:
With head rotated I could feel tight bands coming down to insert on T1-4. I massaged them out some and the snapping greatly reduced. She said her head even felt much better. The muscle must snap over the transverse processes on its way down.