Sudden but not entirely unexpected TWANG
I was so reminded of this in an instant on a EuroManx BAe146, a short-lived shambolic operation about 12 years ago from London City to Isle of Man, operating half a day late. On the descent into IOM there was a sudden and substantial Ker-TWANG from the air conditioning ducts above the seats. There must be something up there in the 146 with a large spring. Everyone around pulled a shocked face, but I just thought "Roger Bacon". Given the overall operation, not even unexpected.
I see I wrote about it here at the time
Reverse Thrust: Vibrations, Shudders, Loud Noise normal?