Whats happening with my pond pump at reduced voltage?
It sounds as though this is a single phase, capacitor start, AC induction motor. Quite common in the 1/4 to 1/2 horsepower range.

When running at full speed, a single field coil is energised at 50Hz (or 60Hz depending on where you are) and this field coil does two things : (1) it acts as the primary of a transformer, transmitting current to the rotor (which acts as the secondary) and (2) provides the magnetic field against which the rotor current's field pushes to turn the motor. 

But to start, this AC field has to act against a stationary rotor, and that imparts no direction to the motor - you could start it spinning either way round equally well.

So to give it direction, a second field winding (the starter winding) is connected via a centrifugal switch and a capacitor; the capacitor resonates with the winding inductance to phase shift the AC current in the starter coil, and this defines the direction of rotation. As the motor spins up to speed, the switch opens, and disconnects the starter winding. This also protects the starter capacitor, which is not rated to survive continuous operation.

So the click you hear is the centrifugal switch turning on again to reconnect the starter. A few cycles of this in a few minutes will probably overheat and destroy the starter capacitor.