What's that rattling noise from parked jets?
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.