That’s interesting about the pilot knowing normal from abnormal vibrations.
I’m a mechanical engineer and have been able to do that in cars and in the house for years. I can hear, feel or sense abnormal operation way before other people. We have a hot water recirculating pump that is hard-plumbed to the copper pipe and the bearings are wearing out. I can hear the low-frequency rumble on our first floor. I can get into a car with worn-out struts or misbalanced tires and feel it immediately. The owner doesn’t notice it because things go bad or wear out so slowly.
I used to work in power plants and we always had the habit of always feeling bearings as we walked by equipment checking for abnormal vibrations.
One of the more interesting tasks in helo maintenance is tracking the main rotor blades, in order to ensure that each one rotates in exactly (or as close as possible) the same plane as the others.