While walking around the airport on a windy day I occasionally hear a tingling or rattling noise from the engines of the parked jets. You can see the fans rotating in the breeze. What’s causing the noise? I should add that the aircraft are not in use, just parked with the engines uncovered.
It's fan or compressor blades rattling loosely in their blade roots as the engines windmill in the breeze. The blades are attached to a hub or disc, and the retaining systems for the blades have to allow for thermal expansion, and also allow for a little bit of sideways compliance as the blades spin around, so they are a loose fit when cold. Compressor blades are usually in a kind of interlocking socket and fan blades are often mounted on pins that allow a bit of freedom of movement.