This is a Kohler one piece quiet flush(until recently). When flushed, everything functions as it should however upon refill, it sounds as though there is air bubbles in the supply line. This is creating loud turbulence noise coming from shut-off valve and fill mechanism- but no bubles emerge.? 3 identical toilets- only happens with one…..Got me stumped.
This Kohler is in a house with no water conditioning system but has a well/expansion tank(new) with bladder. The toilet closest to the tank first made the noise and is now more quiet. Now the upstairs toilet is noisy. There are faucets closer to the tank than both toilets and never any air in water..??
Sounds like you have a problem in the well with a check valve that is allowing water to backflow back down to the pump and allowing air into the system through the pitless adapter. When the pump kicks on the air that has been allowed into the piping is fed into the house piping and the air ends up coming through and into your toilets. I suggest that you call someone who deals with well pumps to look at your problem and do the repairs. Good luck and let us know how you make out. Scott T.