snake "cracking" ?

ok just got home now..

Could the cracking be similar to a human cracking a knuckle or your back? I think so

I agree

I don't think I have ever heard a bone cracking sound from a snake. I have heard what sounds similar when they rub their ventral scales backward as was already noted. They can also make a cracking, popping sound when breathing in deeply with an upper RI.
And rodents do have bones as was noted. I agree that they do seem large compared to their body size and structure.
Thanks,
Rusty

It kinda surprises me that you haven't heard the sound. I hear it more on my smaller snakes. If you try to pop your snake, you will probably hear it when you stretch the snake out.