Jet engines are a class worse balanced compared to aero derivatives operated on ground at least the RR Avon.
Since the mounting are so much more "flexible" in the air is the only reason I got and I for sure believe that.
Humans are sensitive for lo freq. vibration. I have seen people running out of hydro turbine plants at 3 mm/s but low frequency that trigger some escape mechanism I think.
Some designs are obviously passing shell or frame resonances at take off and again I think it is rather low freq. and then you can see it and feel it and yes I guess amplitudes are
pretty hi. I notced that mostly in older Airbus versions, they improved in the newer I think but only subjective it may be influenced by the seating. It may or may not involve the reason for cracks in the beam system holding the wings to the fuselage "pickle fork beams" in quite a few 737NG that was found recently when converting them to cargo versions rebuild certification and quite a few was found at the alert inspection performed on all those flying at only like 50% expected lifetime. Not to mention the classic Hawaiian 737 that got cabbed as a section of the upper fuselage behind the pilots cracked and flew off. If you see thru the wall beside the door when entering the cabin, alert the staff or leave...... That was the case on that one.
You can see nice ODS demos of wing and tail flapping in one of the classic ODS software demos, looks quite nice.
I heard a nice story from CSi sales back in the day where they pulled out their toy 1900 and measured on the DC 9 engines
at their mount from the inside while flying if you were placed in that bad seating you could get a good reading from the aux gearbox on maybe the left engine
and they were as I believed quite different, some did have a not healthy noise if you sat in the seat close. So I tried to avoid that area.