I caved, had to book my golf in for an mot so got the garage to check the audi over. As suspected, they couldn't see anything that would 100% be the cause, and 'of course' it never knocked when they drove it around the yard! so looks like i need to wait for it to get worse. damn. They did notice that the rubbers on the upper arms on one side are showing their age, but didn't have any significant play in the joints. (I did notice the rubbers starting to look a bit old when I looked over it, 'burys head in the sand' at the cost of replacing them!) At least I have a bit more peace of mind knowing someone more mechanical than me has had a look around & not seen anything about to drop.