I'm not a doctor; but does the 'cracking sound' sometimes feel like your pinky is going to break or becomes momentarily painful when this happens? My 'pops' on occasion; and somtimes it feels like the knuckle/joint 'snapped.' Or if I did the lick again, it feels like my pinky is going to break. When this happens, I crack the joint by holding my pinky, and roll it from side to side. This will get the "kink' out of it. I've noticed that this happens when I play a fast run and suddeningly stop and place a quick vibrato (or hammer) on the lick with my pinky. I've also noticed that if I change the angle of my left hand a little bit(in relationship to the strings/frets)it helps keep this from happening.

I would recommend assessing the lick(s) your doing when this happens. It might be the position that your left hand is in, placing a little extra pressure on the pinky. You also might be anchoring your pinky while performing a particular lick, and actually "cracking the pinky" when you execute the lick. For example: Try placing your pinky on a hard surface (like your point it down at the surface). Put some pressure on it, and pivit it side-to-side. If it cracks or feels like it's going to, the vibrato and extra pressure might be causing this. Try different movements with your pinky when doing this and see if you can isolate the causation.

In regards to the one gentelman's suggestion, I don't think strengthening the pinky muscles will help (especially if you can play tunes like Master Puppet.)This sounds more like an orthopedic issue (joints and bones). Specifically, look at the pinky joints (or the knee for that matter)and you will notice that there are no muscles on the sides or tops of these joints. The muscles effect the movement of the joints; but not the actual mechanics of them. This function is assigned to tendons, ligaments and the mechanics of the actual joint -- be it knee, ankle or fingers. Simply put, you can strengthen the thighs and calves from here unto eternity, and still have weak, poppy knees that give out on you when you walk or climb stairs.

I'm not suggesting that you have any orthopedic problems with your pinky, I just wanted to illustrate a point.

I think if you can isolate the movement (e.g., lick or fingering) your doing when this happens, make some adjustments, your problem will subside. If you are experiencing pain and discomfort in the pinky, however, you may want to have it looked at by a professional.