Our Axis has one of these between the 120V side of the water heater and the microwave. The idea (for ours) is that when you use the microwave it turns off the 120V to the water heater. It is used to reduce the total current consumption (both parts when run through one of these can be put on a smaller breaker).
I don't hear it clicking, however, but then I never really try to listen for it (and the microwave is kind of loud anyway). My guess is that the clicking is just a relay inside switching...
Here is the datasheet on the part (pdf).
How it works (from the above sheet):
Quote:
The controls power to two appliance loads. One appliance on the
primary load is connected to the primary output and a shed-able appliance is connected to the secondary
output. The output to the secondary load is carried through a relay.
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