Just because your motor starts to knock...it doesn't necessarily mean that you need a rebuild. It is, however, wise to stop running the motor immediately as to not cause serious and costly damage.
Some times the knocking noise may be your lower unit.
First, have a compression check done on the motor. If that checks out, then pull the lower unit from the mid-section and start the motor, if you don't hear the knocking, you have an issue with the lower unit. You can also pull the drain screw on the lower unit and check for milky looking oil or metal in the oil.