My Coupe Quattro 20v had a bumping sound coming from the engine and my mechanic has traced this to a crack in the worm drive of the distributor and says a replecement Distibutor is required as the worm drive is not available seperately. Is this true? Incidently Audi UK don't have any Distributors and they are on Back order! Is this a common problem at only 70k?