my wrx wagon ticks when at idle or coasting down (in gear, off throttle) i thought it was just a loose heat sheild, which ive since taken off, its's helped but i can still hear a quiet ticking sound.
i was scared it could be a loose bottom end, but had a freind listen to it and he says it just sounds like engine vibration, he had a bottom end go on his old rs turbo and says it sounded nothin like mine

 

when on idle may be the boost solinoid start it and hold your hand against it its on the driverside wing with vac hoses going to it. (cant see you hearing this at motorway speeds tho.