We come across it with our Lifepak 9s.
If I remember rightly, and it was some time ago, we were told it was due to small internal break down of the capacitor and the "cracking" was due to arcing between the plates inside the capacitor. We were lead to understand that the capacitor "self- healed" and so long as the charge time was within spec to continue to use the defib. This phenomenon was due to the capacitor not being fully "exercised" on a regular basis - typical of a 200J regime where the charging voltage is somewhere around 50% of that required for 360J.
You notice the cracking normally during PPM when you take the defib to 360J.
We used to advise our nursing staff to perform the routine at 360J for just this reason - and you could tell who took notice of your instruction by listening to the cap during charging.
As I said, it's some while since I did the LP 9 course and if anyone has any different info I'd be pleased to see it.
Brian
