What is that strange knocking noise?

This is normal. When the sprayer is pressurized, it makes a distinct knocking sound. This is the sound of the of the sprayer’s hydraulics relieving pressure and it indicates that the sprayer is operating properly.

 

When you do not hear this sound, or when it becomes less distinct, this indicates a problem such as the paint bucket being emptied, a tip wearing, the valves wearing, etc.