Diagnosis
Air has gotten into the system, usually in the tubing between the last filter and the tank, or the tank itself has some air in the water side. This is common after filter changes or if the system hasn't been used for a while.
How to Fix It
Let it purge. Open the RO faucet fully and let it run for a minute or two, even if it's sputtering and spraying. Catch the water in a pitcher. The system will push the air out of the lines. The sputtering should stop once the lines are cleared of air.
If That Didn't Work
If it persists, you might have a small leak on the inlet side (before the membrane) that's sucking in air when the system is off. Check all fittings on the feed water side for tightness.
Heads Up
Don't confuse this with the normal sound of the tank emptying. When the tank is nearly empty, the flow will get weaker and you might hear a gurgle from the tank—that's different from the initial air blast from the faucet.