Can you tell where on the engine the clicking or knocking is coming from? Does it sound like the area where you put the oil cap on or deeper down, lower on the motor? Is the noise clicking or knocking . . . like the difference between tapping your fingernails on the hood of your car vs. thumping a frozen leg of lamb on the floor? In any event I think MB is correct . . get an opinion from another mechanic without driving it around, I think that for whatever reason your engine needs to be looked at ASAP. How many miles on it? How was your maintenance? How often did you check your oil and other fluids? Many “off-road” miles on it? Rocketman