What would cause constant bubbling or gurgling noise with the reverse osmosis system?

Answer 1

Some noise is normal. If it's just the sound of water draining into the drain pipe, it's just the way it is. The pipe acts as a resonator and makes the sound louder.

On the other hand, a repetitive surging (glork! ... glork! ... glork!) could be due to debris is the waste line, the large diameter line that runs between between the faucet and the sink drain pipe. Under the right conditions the debris can hold water back momentarily until the pressure rises, then release the flow suddenly causing a noise that gets amplified in the drain pipe. Check the line and especially the outlet carefully for partial blockage, especially bits of gasket.

[Note: Original author posted the explanation that follows, but no longer is sure it can actually fix the problem. It seemed to work at the time, and the theory sounded good, but ultimately the problem was solved by the answer above. It might be worth checking if all else fails.]

It could be air trapped in the waste drain line. Make sure that the waste line goes up from the membrane and continues to go up for at least the first few inches.

Explanation

The waste line has some sort of flow regulator in it, probably a capillary tube, on the membrane housing end. This regulates the flow of waste water, holding constant pressure upstream. If there's an air bubble it will change the flow characteristics, allowing an oscillating flow through the drain. These bursts of water will hit the air gap faucet, flow through the drain tube into the sink drain and get amplified.

Identifying the drain line

It goes directly from the outlet side of the membrane to the air gap faucet. It's a 1/4 inch black tube on an Omnifilter RO-2000, and probably a lot of other units. On that unit it also has a tag that says "Pressure regulator inside".

Re-routing the line

If you have to re-route the line you might have to shorten it. The are two options. One is cutting at the faucet end, which will probably be hard to do in an installed unit. The other is cutting at the membrane end, which means you will have to carefully remove the regulator from the cut section and insert it into the shortened line.