If you have a reverse osmosis (RO) filter system under your sink, chances are you also have an "air gap" faucet on top. If you notice your air gap faucet leaking water through its hole, or if your RO system is making excessive noise, you may have a clogged drain tube. Most RO filter systems need to jettison water as part of the process of pushing it through the membrane. This waste water is put directly into your drain pipe by means of a plastic tube. Since your garbage disposal is also chucking stuff into the drain pipe, sometimes debris can block your RO drain tube.
Shut off the water supply to your RO system under the sink. This is just a precaution to keep you from getting wet, should the system start running.
Locate and unscrew the drain tube. It's probably black. It connects to the main drain from your sink. Place a towel underneath, because you'll drip some dirty water.
Insert a pipe cleaner into the drain tube and push it several inches in and move it back and forth to remove any debris. The debris might be old food particles from your garbage disposal, so be prepared for a stinky mess. If your drain tube is really clogged, twist two together end-to-end tightly.
Reattach the tube securely. Turn the water back on. Check for noise. Note: most household RO systems have a certain level of noise (gurgling, for example) since they discard much more water than they pass as clean.