Diagnosis
That's the sound of water continuously flowing down the drain. It usually means the auto-shutoff valve (ASO) isn't sealing, so the system never stops producing water and waste water.
How to Fix It
First, check your feed water pressure. If it's below 40 PSI, the ASO valve can't build enough backpressure to shut off. You might need a booster pump. If pressure is fine, the ASO valve itself is likely gunked up or faulty. You can try cleaning the valve by removing it (shut off water and depressurize the system first) and flushing it with warm water. If cleaning doesn't work, you'll need to replace it. It's a common part that wears out.
If That Didn't Work
A less common culprit is a clogged or restricted flow restrictor in the drain line. If the restrictor is blocked, it prevents the proper backpressure from reaching the ASO valve. Check the blue flow restrictor fitting in the drain line adapter and clean or replace it.
Heads Up
Letting this run constantly will wear out your membranes and filters way faster and spike your water bill. Don't ignore a hissing drain line for more than a day or two.