only when in low rpms..what is it? it's a 2006 subaru impreza with about 30k miles. The noise feels like it's coming from the passenger side but under the hood. I give it a lil gas and the right side rattles. when i'm driving or when in idle. when i give it more gas it goes away. but will always come back in low rpms. This started yeterday from the crappy weather. any ideas? heat shield? something loose? something rusted through??