Clicking noise from engine

If you can't describe the noise by pitch, loudness, whether it changes in speed with changes in engine speed, what area it's coming from, whether it occurs all the time, just at certain temperatures, etc, you'll need to have a mechanic listen to it. There's dozens of things that can be described as a click and some don't even require the engine to be running. Some emissions devices click at a rate that has nothing to do with engine speed. Some of them make noise only when the engine is running. Some can make noise even with the ignition switch turned off.