Probably a dead battery but you may wish to check your battery terminals for corrosion.
Low Battery
clicking noise when trying to sart engine usually indicates a flat battery.if the battery is not the problem ,the starter motor needs repair.
A clicking sound, when trying to start the engine, indicates a dead battery or poor (dirty) battery or starter relay connections.The clicking sound is the starter relay(soleniod) not getting enough power to hold it's contact points together.
Does the engine turn over and then make the clicking noise while trying to start, or does it not do anything but make the clicking noise when you turn the key. If its not turning over at all then it might be a battery problem, Try cleaning the battery terminals, and/or charge the battery, Could also be the starter solenoid, but start with the battery first.
it might be your starter.
Maybe flat battery?
The clicking sound is probably your solenoid trying to work. Your battery has just enough electricity to make the solenoid operate but not enough electricity to make the starter work. You probably have an almost dead battery.
battery is FLAT insufficient power to hold the starter solenoid in to start /turn engine over
Check the wire from the battery to the body and from the battery/body to the engine block.
If clicking while trying to start then might be a bad starter. If clicking while running, then might be valve/lifter noise.
On a 2001 Ford Taurus : In the Power Distribution Box ( which is " live " ) located in the front of the engine compartment , near the battery ( mini fuse # 18 is a 20 amp fuse for the cigar lighter )
A weak battery. There is not enough voltage to hold the starter solonoid in.