Update!
I've tightened all the screws holding the plastic window in place and it seems to have solved (or very much reduced) the cracking noises.
I'll see for sure later tonight when I have more time.
Nice tip!! Will try this on mine over the weekend. Mine does do this occasionally, annoying as I jump as soon I hear it and give the whole PC a good inspection. I think it's where its so occasional then nothing happens that I kind of ignore it once I've gave the case a quick check.

