Should I be concerned by DS2's (he's8) cracking knee joints?

[saltire]

and the fact that he says they really hurt when they crack?
I heard them for the first time a couple of weeks ago, when he sort of fell over on the carpet. He said they happen a lot and hurt. Then today I heard them as clear as anything, really loud cracks. I can pinpoint it as starting to ahppen after he had slapped cheek virus. He got the rash from it all over his body, and his joints all swelled up and he was in a lot of pain, and the last areas from which the rash went were his knees and wrists.