More then likely as stated few times,
injectors or lifters. If its freaking you out, get it checked by a reputable mechanic but as far as i know its totally normal. My bosses S5 is crazy loud, hes had it since new, babys it around and its only done 10,000kms. Audi has done all the tests and has forked out for various workshops to check it and put his mind at ease.