Cracking sound from double glazed window during storms

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So here's the problem. Whenever there's a South Westerly gale force wind, the master bedroom window (possibly window frame) makes a loud cracking sound. The window surround is made of uPVC, and the window is double glazed. This has been going on for a while now, and it drives me nuts whenever it happens.

Has anybody got any ideas what could cause this, and what needs to be done to fix it?