Loose blades
I was in the Deutsches Museum in Munich today - it's excellent, and I can definitely recommend it.
They have quite a few turbojet engines with cut away sections so the interiors can be seen, and in a few cases (such as the RR Pegasus) touched. I noticed that the compressor and turbine blades on the Pegasus were all slightly loose, and could be rattled by hand. Does anyone know why this is? The only reason I can think of is that with thermal expansion at normal operating temperatures they become tight.