When a cat makes a clicking noise while purring, it could be an indication of a behavior known as chattering or chirping. This behavior is commonly observed in cats when they are watching prey, such as birds or squirrels, through a window. The clicking noise is often accompanied by rapid jaw movements.
Chattering is thought to be a frustration or anticipation response. Cats may make these clicking sounds as a result of the excitement and frustration of being unable to reach their prey. It's believed to be an instinctive behavior that mimics the sounds cats make when they capture and kill prey.
Some experts also suggest that chattering may be a way for cats to practice the coordination of their jaw and mouth muscles. It could be their way of preparing for the precise movements required for catching and killing prey.
Overall, the clicking noise during purring, known as chattering, is a fascinating behavior exhibited by some cats when they are stimulated by the sight of prey. It's a natural instinct that allows them to express their excitement and prepare for potential hunting opportunities.