The fuel solenoid clicks in the on position
The starter solenoid clacks in the start position
The PTO solenoid clunks when PTO switch is turned on.
A continious clacking will usually be the starter solenoid in which case.
Battery voltage being applied to solenoid is too low to maintain contact.
Your battery is bad or the connections between the battery & the engine are bad.
Remember there are 2 connections that need to be clean & tight, one at the battery and one at the other end of the wire.
A single light click is likely to be the fuel solenoid, remove it from the carb and try again.
No click = faulty starting circuit
