What is that hissing sound?

When the RO/DI is on, the unit will hiss. That sound means water is going out the waste line. When the product line is shut off with the ball valve (on the RO or DI line - if you use one for each, both must be closed), the unit will pressurize and should stop hissing within 30 seconds. If it is still hissing, water is being wasted. Turning off the water before the unit is another option if it won't shut off correctly.