scratchwin

I have an irritating noise when accelarating from low walking speed to 30MPH, coming from behind dash. Thought it was atwig around the axle. Sounds like a bird dancing, a type of scratching noise, sometimes like a plastic ball rolling around. Sometimes it appears when starting up. Could it be something loose be in the aircon ducts, or behind the radio? It echoes a little so is difficult to locate. Any ideas?