I've been getting a randomly appearing really loud clicking noise that increases with rpm, but seems the click seems to occur every 0.5 a second or so when the engine is at idle, I did a snoop one day for the noise and i think it is coming from the front of the engine that is to say the accessories and not the valvetrain. today it came on when i started the car and after i drove a few kms it went away, but on the way home this afternoon i only noticed it halfway through the trip, only on idle then it disappeared when i started off from a set of lights.

Is it possible that it is the mechanical fan making this noise or one of the other accessories. i suspect a sticky lifter but it seems to happen so infrequently and randomly that i am doubtful that is it.