(...) It's the only transformer I've ever held in my hand that rattles... And apparently it's the way Neumann did it and should go away once mounted? Magnetostriction...
As I understand it, magnetostriction is only present in ferromagnets- those that remain magnetized at all times (i.e. permanent magnets).
Transformer magnets, as used in condenser mics, are not ferromagnetic, so that phenomenon would not be encountered there.
I would therefore question your use of the word "apparently".
Besides, I have never run into rattling magnets in microphones, new ones or old ones, aside those with badly constructed or sloppily assembled transformers. Which would indicate that, whatever rattles in yours will not go away with time.
I would suggest you inspect the transformer closely for manufacturing slop.