Most likely a faulty relay, possibly a bad earth, or your alternator may be overcharging.
Check all earth cables, especially the engine to chassis.
Get your battery and alternator output checked. Places like kwik-fit, halfords etc used to offer free testing, or just buy a tester from Halfords, they are cheap and simple to use, just clip on to the battery.
If it's not that then it is probably one of the relays failing, that is the clicking noise you hear. You can have a look yourself by removing the cover and feeling for a clicking or hot relay, but you may have to get an auto electrician to check that.
I suggest you do the free checks first.
laguna, November 2009