What could be wrong if the motor is knocking?

Most likely is con-rod bearings. If it is, they will gradually get worse. It is best to change them before they fail altogether. Take the car to a reputable mechanic for an opinion first. Not being able to hear it I may be wrong, could be tappets needing adjustment. However,experience tells me out of adjustment tappets tap noisily, it is worn bearings that knock. or you could just be low on oil