Okay First, You want to learn and continue to perect how to do a pull up. Number of reps is secondary. Form is primary.
If your back is rounding and you're not pulling with your lats, than you are going to use shoulder/rotator cuff muscles and brachialis(arms) to execute the exercise. Always engage the scapulas and don't let them disengage at the bottom. Doing so helps protect the shoulder joint and like the video shows could allow for unneccessary extension of the neck. (Nerve pinching)
Keep the upper back pulled back (engaged). not rounded and pulling with arms and shoulders. In other words, Imagine bringing your elbows back and towards your side. practice up against the wall raising and lowering your arms with your shoulders pulled back (wall angels)
The cracking is probably due to the joint irritation or the tendons snapping over the joint. in either case it's not good to continue doing the same movements. long term irritation could lead to irreversible damage and unneccesary complications.
post a video of your pul ups
Best of Strength to you ~