Is the clicking coming from the horn itself? If so, then a few possibilities spring to mind.
(1) The horn is knackered, and needs replacing.
(2) The voltage is too low to operate it. You need to disconnect the horn connector, and check the voltage when the button is pressed.(i.e. live terminal to bike frame, with meter set to dc volts), then ground side to bike frame (no need to press button), with meter set to ohms (there should be little or no resistance)
(3) high resistance caused by a poor connection in the connector. You need to uncouple it and clean/dry it.
Carry out these tests and let us know. If you haven't got a multi meter (volts, current, resistance) you can buy one very cheaply, and it'll be invaluable for future use on the bike. Without one, you're just guessing. :2tup:
BTW:- If you do need a new horn, and decide to fit a different one, check out the reports at
http://www.webbikeworld.com