DIAGNOSIS Air trapped in the filter housings or a vibrating check valve inside the unit. The gurgle is air moving, the hum is a valve resonance.
FIX For gurgling, bleed the system. Open the filtered water faucet fully. Open the blue bypass valve slightly until a steady stream comes out (catch it in a bucket). Close the bypass. Repeat a few times over a day. For humming, tighten the mounting bracket screws to secure the unit firmly to the cabinet wall, which can dampen vibration.
IF THAT DIDN'T WORK The noise might be from a partially closed shut-off valve upstream. Ensure the saddle valve or inline shut-off is fully open.
HEADS UP Some low humming is normal from the flow control valves. Loud rattling or banging means there's a loose connection or water hammer—install a water hammer arrestor if needed.