Its the solenoid clicking in and out (in on the key, out as the starter motor drops the voltage, then back in after the solenoid drops away, repeat). If it isn't the battery knackered, then it must be either the positive cable from the battery to the solenoid or the earth strap(s) from engine to battery not conducting properly (e.g. broken, corroded, frayed, loose). My bet is the earth strap side, as it comes up over and over again. Can you start it by jump leads to your battery? How about +ve jump lead on your battery, -ve lead attached to your engine?