Great to hear you started the 14 day challenge.
Has the ever happened before?
Crepitus (creaking, cracking, grating, crunching, or popping that occurs when moving a joint) without other symptoms is ok usually is not a cause for concern.
Many people’s joints crack or pop occasionally, and that is considered normal.
Air bubbles popping inside the joint could be happening.
You mentioned lacking significant mobility when doing the Phelps this may be coming into play.
Keep an eye on it as you are working on your overhead mobility.
Note better, same, worse as you do it.