Interesting comment on the vibration. I only flew on two legs as a passenger and sat between the engines each time. I recall a lot of shaking in the cabin, but nothing specific.
On the other hand, in cruise the props were out of sync most of the time. This would cause an area of about 2 square feet of gravel, dust, dead bugs, bits of candy wrapper, loose washers, and who-knows-what-else to vibrate from one side of the aircraft to the other. Some of the stuff would get airborne about a half inch off the floor.
As I'd watch this collection of debris buzz its way across the floor from left to right, I could see it coming my way, and feel it in the seat .... then it would travel into my ear canal and felt like there were a hundred bees trapped in my head!
A few seconds later, it would travel across the cabin in the other direction.
Made me wonder what was happening to the innards of the airframe over thousands of hours.
Regards to all,
Fog