Ah, the singing tip. We had a singing props that supposedly had ati-singing edges. When we sent a diver down, the edge was ground on the wrong side of the blades.
We had a severely vibrating engine.
The crew swore up and down it was getting worse, to the point it was felt in the Captain/Chief Engineers cabins. We checked everything, dampers, fuel timing, it was to the point that we were starting to think the rubber blocks under the engine bedplate were starting to fail allowing more vibration.
After all of this work we had a Chief return back to the ship after 8 years away and he started looking around the engine. He noticed some minor deformations in the gussets and tanktop, that had been overlooked and he theorized that the forward edge of the engine seat could have been slightly depressed. (Think of an engine on a raft). He put a hydraulic jack in under the seat and just pumped it up enough to make contact and the vibration stopped. It appears that the location of the engine and the ship structure were transmitting the vibration up 5 decks to the point everyone was expecting the engine to disintegrate.
Om another ship we had a knocking coupling. It only did it on certain low RPMs though. At the end of it, everyone assumed that the locking plate was off inside the coupling and knocking. With the ship due for dock, it was left.
When the coupling was split, it was found the locking plate was still bolted in place, however the holes had elongated, allowing the plate to rock as the shaft tuned- the source of our knock.