In these cases the vibration was felt by body parts rather than heard. One time I commented to an FA (from my comfy semi-supine position) who (standing) agreed that she also 'felt' a vibration unusual enough that she communicated it to the flight deck.
On that particular flight the a/c had emitted a god-awful screeching sound for perhaps 30 seconds during climb-out so one could imagine a loose panel involved.
Again, no expertise claimed....
pj