Is this the first use of the machine? Is the water tank empty, or has the machine been allowed to sit for an extended period of time?
If there is water in the tank and this is the first use, the knocking sound is normal. This is the sound of the pump trying to pull water through the machine. It will happen on the first use, each time the unit runs out of water, and if it sits for extended period of time allowing the water to evaporate.