While there are several different designs out there for turn signal switches, they classically used some form of a thermal switch (in this case called a thermal flasher). The core of this switch includes a resistor that heats up when current flows through it. This is attached to a bimetallic spring- essentially an arched strip made up of two sandwiched metals with very different thermal properties, one that expands quite a bit as it warms and the other that doesn’t….So in these thermal flashers, the clicking noise you hear is just this bimetallic spring snapping back and forth from its arched form to a straightened form and back again. …Another common device used to create the blink in your blinkers is an electronic flasher. Instead of using heat and a bimetallic spring, this device simply uses some electronic circuit to control a relay. Inside a mechanical relay, applied electricity to an electromagnet creates a magnetic field that switches a metallic armature, which in turn makes an audible clicking noise.