Not knowing the mileage, condition of vehicle or circumstance I am going to try to list things from easiest to difficult. These tips are dependent on what area of the engine the knocking noise is coming from. If this happened after an oil change, you may have a bad or incorrect oil filter. There are valves in the oil filters and if it is bad or incorrect it can cause an oil pressure loss, so you can either lower engine or valve train noise. A belt tensioner that is seizing up can make a knocking noise. If the noise is from the top of the motor and louder at one side there could be a timing chain problem. Another source can be a cracked flex plate,( also called a flywheel) the transmission would have to be removed to inspect this. On high mileage and poorly maintained vehicles, carbon build up on a piston can cause a knocking noise, but this is very rare these days. I am assuming the oil is the specified weight, the level is correct and the oil pressure gauge is accurate, if they are not, then you can have something odd like a cracked oil pump pick-up tube. This condition would cause low oil pressure. Make sure what you are experiencing is an engine knock and not pinging. Pinging is the pre-ignition of the air fuel mixture and is usually noticed under load. If you have pinging all the time you may have a fuel delivery problem or vacuum leak, but these conditions should cause the check engine light to come on. if it is pinging you could have a plugged fuel filter, restricted fuel injectors, poor fuel pump delivery volume, even bad grounds can cause this. I hope this helps you.