Hard knocking noise
 

I just purchased a 1997 audi a8 in mint condition but the problem is the engine is knocking hard and loud when u start up and doesn't go away. My questions are 1. Is there a cheap fix for it or should i replace the engine? 2. Can i pit a 98 audi a6 engine into the car? 3. Is there a site that sells cheap engines? Pleaseeeeeeeeeeee hellppppppppp!!!!!!!!!!!!!!

Mint condition is relative term...these engines are generally bullet proof (high miles?).

If you lack the facilities, tools, knowledge, you need to get it to a mechanic to find the cause...NO way of telling from just that description. Need to do a compression check, check cam timing, depends where knock is coming from, etc... Cost would depend on the problem..in the block, in the head(?). Not likely to be cheap.

Find the cause before you start engine shopping. Interchangeability depends on the engine you have now and the exact engine you may swap...plus a matching ECM (engine control module) may be required, if engines are significantly different.

Hard to say exactly what's wrong with your car. A hard, loud knocking from deep within the bowels of your engine, would suggest conrod bearings, or crankshaft main bearings.

I wouldn't drive the car in that state - if it's the conrod-to-crank bearings, high rpm could cause catastrophic engine failure. If you throw a rod, you'll end up with a window in your block, pretty terminal if that occurs.

Get your car to a shop that knows their stuff, before looking to buy an engine. And just for reference, the Audi A8/S8 D2 on a '97 model year will be a 32V engine. The Audi A6 C5 4,2 never had the 32V motor, from the onset, it was fitted with the 40V unit. Whilst it will physically fit your car, it won't run on your ECM and harness. You'll need to fit the harness from a '99 A8, matching ECM and TCU, then code for IMMO to get it all to work.