Update. Been on support call all day to Apple, despite me saying this is an issue others have had and that they've tried the same you're making me do, I've gone through an SMC and NVRAM reset. I'm now going through the process of reinstalling macOS. Once this is done they'll finally have an engineer look at it.
I'll be asking for a straight swap. I don't want to be without my two-week old laptop. Alternatively, I'll live with it until someone else finds a solution.
macOS is now reinstalled and there was also a supplementary update available for me (assume this was the throttling update). I'm pleased to say that under conditions where the crackling would occur, they've not since I've done this.
Stress testing for the last hour and so far so good.