C550's do tend to crack a little most of the time, some do, some don't. Again I believe this is the pressure bulkhead adjusting from my experience's.
Occasionally in one of ours it makes an almighty Bang and scares the **** out of me every time ! used to do it all the time but sems better now. Don't ask me why ?!
Some XL's I've flown tend to crackle on the way up through the mid 20's aswell.