If this is the classic engine knock, that can be corrected by the typical fuel and cumbustion related remedies, and it may be worth doing anyway to rule those things out. But if it isn't that at all, then one's mind unfortunately goes to mechanical breakdown of the crankshaft components, journal bearings, main bearings. Even the transmission should be checked for the noise. I have a vehicle that is believed to have a wrist pin noise that is only coming from one cylinder, but it is heard from the upper part of the engine, so I don't expect that to be your problem. You need to have it listened to from below with a mechanics stethescope or like to listen around for the source of your knock. Sorry, it doesn't sound too good to me from here
