The knocking noise is the normal sound of the pump trying to pull water through the machine. It will happen on the 1st use, each time the unit runs out of water, and if it sits for an extended period of time, allowing the water to evaporate or if they are using a new filter.
If this is the first use of your machine, hold the trigger down until the noise goes away for up to 1 ½ minutes…
If this is NOT the first use of the machine, then water is not able to get through. Ensure there is water in the tank and that the filter is seated properly with no gaps.
If the unit sat for an extended period of time, hold the trigger down for 30 seconds as this should resolve the issue.
If you have further questions regarding this, please call us at 1-800-237-7691, Monday - Friday 8 a.m. to 10 p.m., or Saturday 9 a.m. to 8 p.m., ET. A representative will assist you.