Refer to figure.
The rotor blades of an axial flow compressor are held in position by the mountings, the aerodynamic force, and centrifugal force. The mounting connecting the blades at their root is of the ‘fir tree’ type. The blades are subject to very high centrifugal loads and high-temperature variations that unavoidably cause thermal expansion of the blades. That’s why they are fitted on a disk or drum using a ‘loose fit’ design that will avoid them blocking in their mounting under high thermal loads and vibrations.

The ‘fir tree’ root connection allows the blades to be loosely fitted and to be replaced more easily, as each blade has its own independent connection to the rotor disk. The loose fit gives the rotor blades some room for expansion. Sometimes when the aircraft is on the ground and a strong wind is blowing towards the engine and keeps it rotating along with the compressor (the engine is windmilling) you will be able to hear a clicking noise. This is due to the rotor blades being loose in their mounting on the drum without any centrifugal loads acting on them.