Every two seconds? If it was the same as engine speed we could look at the rotating mass, half engine speed and maybe lifters and cam associated parts..but every 2 seconds? Are you sure its not more frequent? I mean are you just generalizing with 2 seconds or is it really? Does it increase with RPM or go away? Also are we talking about two different sounds? You called it a knock then a tick, in engines those are two completely different sounds and one is way worse than the other. A tick can be any number of relatively harmless things from piston slap, injector noise, very loose timing belt, or lifter tap. A knock is generally much worse with things like rod knock, excessive crank play, worn wrist pins or a host of other "needs rebuild" problems.
This is rod knock: http://www.turbo4wd.com/movies/EngineClanking.mpg
Generally not something at idle but if its bad enough it will. Give as much detail as you can about the noise and it may be easier to diagnose.