Gurgling is usually just trapped air in the lines, especially right after installation. It's common.
Let the system settle. Running the cold water faucet for a few minutes can help purge the air. Air bubbles can cause cloudy water for the first day or two, and that same air can cause gurgling sounds.
If the gurgling is accompanied by very low water flow, you might have an airlock. Run all your faucets—hot and cold—and any side sprayers to clear it.
If the noise persists for more than a couple days or starts to sound like a serious rattle, check that the unit is mounted vertically and all connections are secure. If you're unsure, a quick call to APEC Water support can rule out a faulty valve.