SSR ticking when controlling heater
It sounds like your SSR is missing half cycles.
The ESP8266 is a 3.3 V device (though you may be operating at less than this), and this supply may be marginally low for turning on your SSR.  

Make sure you have enough current flowing in the LED side of the SSR, and you probably need at least 4 V or more to ensure your turn on.

You are using a Zero crossing SSR so you can't PWM it of course. 

Datasheet