I just had to look up the term 'magnetostriction'. It could be that, but I doubt it. I reckon that the transformer windings, which should be securely mounted on the transformer core and fixed in a resin have just loosened for some reason, possibly bad manufacturing, or old age or tail plane vibrations?
Aircraft AC is 400Hz of course, but the panel the faulty transformer was attached to could in theory surely resonate at any frequency mathematically related to that - simple harmonics?
U