Thanks for answering!
No, it's not trim, that doesn't bother me at all and I've heard windowns 'settling', trim creaking, trolleys banging and people who clap their hands onto the bubble packs of cutlery to make them 'pop' open - though that really got me first time... This sounds significant and structural - though that, when I read it back probably makes me sound a bit of a 'loon'.
Again, it's prob to do with that awful Viscount experience when on 2 ocasions, weeks apart, there was a sound like cables slapping on bulkheads or underfloor on take off that transpired to be the cracks opening and closing, if I recall correctly - just prior to grounding the fleet.
I've tried to find references to that but can't, though it's burned into my memory.