And those are probably all “normal” vibrations; over time, a helo pilot learns to detect the abnormal vibrations, which come in three varieties: Low freq, med and hi-freq, and their causes. To fly is heavenly; to hover, divine.
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.