Loud engine knock

Some STP might help diagnose the problem but as far as fixing it, you might need to replace a rod bearing, etc. The STP is really thick and will help create a cushion between the crankshaft and the connection rod bearing, where the motor oil is and help quiet it down but that will not fix it. If the STP quiets it down and the noise is coming from the bottom and it is a knock, then it might just be a worn connecting rod bearing.
You could also even go beyond that and try a cylinder balance test and cancel out each cylinder one at a time and listen for if and when you cancel out a cylinder, that the noise changes pitch. If it does you could also narrow it down to which cylinder connecting rod bearings it is, so you will only have to inspect the bearing on that one cylinder. Of course you would have to do the balance test before using the STP so you can still hear it.
It is not hard to change a rod bearing, after removing the oil pan, turn the crankshaft over until that rod is down, remove the rod cap, use the wood end of a hammer or something similar reach up onto the bottom of the piston and just tap it up far enough that you can slide the top bearing half around and grab it and remove it. Make sure the crankshaft journal is not ruined with a lot of deep scratches. Install the new bearings with a little motor oil on each outer surface, but you may want to use some plasti-gauge first just to check the gap and if so wipe all the oil off, because it has to be dry. Do not forget when installing new bearings to smear some motor oil on the new ones, install them in the rod and cap, pull the piston back down, install the cap and torque.