Rapid clicking sounds from the starter indicate a low battery.
The starter motor requires a large amount of current to crank the engine. The current requirement is too large for the ignition switch. Instead, the ignition switch operates a solenoid which engages to supply current to the starter.
What is occurring in your case is that the solenoid engages. This connects the battery to the starter motor. The starter motor loads the battery causing the voltage to drop. The reduced voltage is insufficient to hold the solenoid and it opens, disconnecting the battery from the starter motor. The battery voltage recovers allowing the solenoid to re-engage and starting the cycle all over again.
It is this cycling of the starter solenoid that makes the rapid clicking sound you hear.
The fault can either be a dead battery or a charging problem. Batteries degrade over time so the battery is the most likely problem if it is a couple of years old.
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