It's called "gettin old" dude.
But seriously, I went through a stretch of a couple months where mine would stiffen up rather painfully, and I would have to force the pop to relieve the pain. I don't know for sure if that's safe or not, but now they do it all the time without the pain.
I tend to think it's similar to cracking one's knuckles.
TuG.
Embrace your enemy, for he is not - he is just confused.
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"It is very hard to unleash the dragon and then put it back in the box without being devoured in the process."
-Sifu Abel