Your random misfire could be a number of things.
1. It could be that the engine is running to fat and spitting way to much fuel. This will also burn up your cats, if the car has still has them. Keeping them on a FI car isn't a good idea. You will eventually burn them up.
2. It could be a bad plug or ignition coil. These will cause your car misfire and could be the reason you hear a knocking sound.
3. It could be dirty oil. This will cause your timing valve to get stuck open. When that happens, hope you have a 10mm, a rag, a small screw driver and about 30 minutes on the side of the road to fix. FI cars will burn through the oil faster and dirty it up quicker since it's doing double duty. I was changing mine about every 2000 miles.
These things I know from past experiences and they are all simple fixes. If the knock isn't any of these things then it could be that last owner ran the sh*t out of the car, knew it was about to blow up and sold it off before he had to pay to fix it.
Good Luck!
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