Yeah, this one's common. It's usually the drain line vibrating against the sink cabinet or pipe.
The black drain tubing has constant pressure inside it, which can cause it to vibrate. The fix is to secure the tubing so it doesn't rattle.
- Check where the black drain tube runs from the RO unit to the drain saddle under your sink.
- Use zip ties or adhesive clips to fasten it to the cabinet wall or another stable surface, keeping it away from other pipes.
If that didn't work, the noise might be coming from the drain saddle itself. Make sure the drain saddle is mounted as low as possible on the vertical tailpiece, or on the horizontal tailpiece, to reduce drainage noise. Also, ensure the self-adhesive black sponge is correctly installed between the saddle and the pipe to cushion it.
If it's a loud, new grinding sound, check that nothing is blocking the drain flow. A clogged drain saddle hole can cause back pressure and abnormal noises.