After driving my astra at high engine speeds (4-6k). When starting the engine (particularly when hot) I hear what sounds like a rattle from the exhaust or something in the engine bay. This goes away as soon as the engine has started. The sound only happens again when moving off in 1st. It also happens again when I labour the engine (below 1500rpm). I'm running the car on v-power so I don't think it's engine knock.