We had a similar problem with an E46 BMW. In that case, I searched the area nearby for a place that would create the same 'knock' but even then the mechanic had to get out of the car because the extra weight meant the problem wasn't being recreated. In the end it turned out to be a lower control arm. Can you do something similar to try and recreate the problem your Audi is having? It sounds like you've investigated the usual suspects but don't forget the antiroll bar links if fitted and the engine mounts (I'm assuming there's nothing loose in the interior). If you can't live with it, you've got a few choices: take it to another mechanic, a full suspension refresh (expensive) or just hang fire until it gets worse - once it reaches a point where it's something has obviously failed you'll be able to spot it and sort it. Not ideal I know, but might save you a lot of hassle (and money) chasing a hard-to-find fault.