9 times out of ten, its the "element"
To complex to tell you how to check, in a news posting, but if element is bad,
probably as cheap to replace whole heater.
(what is the LENGTH of the heater? Is it 110 or 220 volts?
Click sound is control or thermostat making contact
If you do try checking your self BE CAREFULL
Some 220 volt units have a live wire still on, even when wall control is turned OFF