The clinking noise is caused by the turbines.Under normal operating conditions The attachments between the turbine blades and shaft are very hot and is spinning very fast so an expansion gap is left. This causes the individual blades to be "loose" when cold and turning very slowly. Thus the clinking noise is when each bade reaches the top of the circle it drops down towards the shaft, and at the bottom of the circle it drops away from the shaft.
In general it wont do any harm to the engine over short periods of time. However stored aircraft would more normally have covers fitted, both to stop windmilling and prevent animal infestation, sand or dirt.
Hope that helps.