Perhaps the contacts in the plug aren't making contact securely and are repeatedly closing and opening the circuit through a low surface area gateway, leading to audible clicking as a result of sparking. Over time the charging contacts may have become increasingly worn to the point they do not make any connection at all, stopping the clicking and the charging.
Usually I'd expect more current/voltage to hear this phenomenon, but it doesn't seem terribly unfeasible that it would happen with a high voltage laptop charger. I might be extrapolating too much.