annoyin noise (ticking or rattling) on idle..

[abdallah380]

hey ppl, my yaris runs like a dream, but there is this annoying ticking or rattling noise not sure how to dexcribe it that can be heard when the engine is on idle. even after the warm up. it can not be heard when i accelerate or while im driving only when the car is parked or lets say stops for short at red lights the noise is clear.
the sound is very faint, but when i have my a/c on the sound can be heard loud enough to annoy me. the sound clearly is coming from somewhere under the hood.. any suggestions? thanks :)