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An A4 is a classic industrial engine. If the knock is something sinister like a "od knock" indicating death and disaster are soon to come...there's no reason a local shop can't do a proper rebuild. Any COMPETENT engine mechanic can do the rebuild, and most handy boatowners could actually do it--given the time and space.
If you can't get a local referral to a good engine shop (one where the shop is fairly clean and the folks aren't shouting and cursing all day) ask a local NAPA or AgWay dealer if they know a good engine shop or tractor engine shop.
There's nothing fancy in an A4, it is just a classic gasoline engine still using 1950's technology and running at such a low compression ratio that it can run damn near forever.