Cracking noise in double-glazed window. Why?

[Glug]

Three years ago I had some new UPVC double-glazed windows installed. Any help identifying cause/problems much appreciated.

No 1: A tilt and turn window, with a small side opening window, both part of the same unit.
Since installation the tilt and turn section sometimes emits loud cracking noises when closed. Is this due to temperature difference between interior and exterior panes? It didn't happen with the previous UPVC window.
Also, there are two cracks in the plaster where the unit is installed. A vertical crack 2mm all up one side, the other is horizontal up to 3mm all the way along the top of the unit.
I have two other stand-alone tilt and turn windows and they are not doing this. Is the unit too heavy and is it about to fall out? Should I be worried????

No 2: One of the external vertical seals on another window turned mustard yellow. The seal was replaced and turned yellow almost immediately.The other vertical seal is not affected. Does this indicate damp somewhere?

Thanks for any advice.

 

[The Bullet]

No 1: Does the tilt and turn "crack" when you are turning the handle to lock, or just randomly cracking when it's shut?

No 2: You should use silicone, not mustard to seal windows.

 

[God]

Mmmmmm mustard

 

[Glug]

Thanks guys. I'll leave you to talk among yourselves.

 

[scotchargus]

I had double glazing installed for the first time October last year and I can hardly sleep for my bedroom window pane making loud clicking sounds every ten minutes or so. I thought it was coming from the wood surrounding the pane but it's coming from the glass.