Diagnosis
This is usually not the pump. It's water hammer or vibration from the fast-closing solenoid valve in the automatic shut-off valve assembly.
How to Fix It
Check that the feed water supply line is secured and not touching the cabinet. Add a water hammer arrestor to the cold water line feeding the system. A small 1/4" inline arrestor installed between the angle stop valve and the RO inlet can absorb the shock.
If That Didn't Work
The drain saddle clamp might be loose, causing the drain line to vibrate. Tighten the clamp on your sink's drain pipe. Also ensure the drain line has a high loop secured above the drain connection to prevent back-siphoning.
Heads Up
A constant grinding noise is different. That could be the booster pump failing. If the hum is only for 2 seconds when the tank fills and stops, it's normal valve operation.