Landroger
I think where I have a problem is with the sheer mass of either rotating parts or, more probably, the air being moved by these huge donks. A friend of mine supplies quality sound equipment for professional 'gigs' and I watched him 'running in' a couple of speakers each about the size of a Mini. He had placed them 'face to face', up tight, in his living room! He was feeding an out of phase 'beat' frequency into them, at pretty much the full output of his amplifiers which, I can assure you, can move your internal organs.
However, virtually no sound could be heard. Move one of them, at an angle, an inch away from the other and the furniture started to move.
If a beat can be heard in an aeroplane, it means that the vibrations created by one engine are not being cancelled by the other and that energy
may be being absorbed by the airframe. Could this not produce stresses in the airframe, particularly if the perceived beat frequency hits a resonance?
Of course you are correct, for even I too have run such home experiments and broken the crockery.
But aircraft structures are not not that sensitive to what we percieve in our ear. They are incredibly well dampened to vibrations by virtue of their many joints and bonds let alone their myriad of frequency responses that cancel each other much like the clothing that we wear.
It's the single structure (without joints or bonds) that amplifies this noise to our ears in an airframe. Such things as interior wall surfaces (nomex?) and in some cases the skin between ribs (drumhead effects)
Fortunately we don't have aircraft falling apart due this effect in the data base (not withstanding the unimportant stuff in the cabins
)