squeaking noise from WRC cars

[anstis]

And back to the original question, where does the squeak come from?

Don't quote me on this but I think I read somewhere a long time ago, from memory it might have been David Lapworth explaining, that when the throttle is closed the compressed air, having gone through the turbo, almost stops as there is less being fed into the engine. The compressed air then momentarily expands forcing some backwards through the compressor wheel as it's still spinning, creating a chirp.

I could be mistaken though...