The rapid clicking sound you hear is the starter relay operating then dropping out then operating and dropping out again.
The battery has enough energy to operate low current devices - like the boot and the starter relay but as soon as the starter tries to draw 50+Amps to turn the engine over its voltages drops to a very low level - not enough to hold the starter relay which disconnects the starter motor - and suddenly there's enough voltage to operate the relay again - click-click-clickety-click.
Not enough energy in the battery to turn the engine over - either charge the battery of get a new one.
Good Luck...
March 2016
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