Fabiasi

My wife's citigo has developed a rather strange noise from the engine. It is a continuous woooooooo noise and if you place your hand over the air intake of the airbox it stops. Again if the little pipe from the airbox is removed and covered with your finger it stops. My thoughts were that it was the PCV valve in the rocker cover but I have replaced this and the noise is still there ? It definitely seems worse when the engine temperature is cold. Has anyone else had this or is it possible it's something that we just haven't heard before ? Thanks Simon