Well, the noise happens kind of every so often, and the crunching doesn't happen every time i brake. Just sometimes as well. I've got a 20 minute drive home later...not confident of making it.
(i'm jealous of your Audi aceness).
Hard to say then really, might be something as simple as needing new brake pads, that's what the crunching sounds like it might be. Could well be something else though. Nailed on it won't do it when you take it to a mechanic as well. Bloody cars! Get it in to a garage would be my advice. Sorry I can't be more helpful.