How do you stop a loud horn-like noise that occurs after you flush the
Make sure the shut-off valve under the toilet is open all the way. You are experiencing “air hammer” which is usually caused by a leak drawing in air when you use a fixture. You could also have your plumber install an air chamber on the shut off under the toilet to prevent this problem. These air chambers that cause this loud hammer like noise or horn noise you describe are not uncommon. It is a phenomenon that occurs when water pressure and air pressure are not in balance in your plumbing lines. Imagine a freight train, your water, running through your pipes at a high speed. Then suddenly it runs into a wall of air pressure and you get the noise that is akin to a train whistle as the water slams do a stop. Sometimes people hear this plumber pipe noise as a groaning noise or a knocking noise and it can radiate all around the house. Air hammering in the home can be sometimes addressed by ensuring your plumbing pipes are securely in place. The knocking pipe noise that you sometimes hear in your walls are up in the attic is also due to this air hammering effect. It is not always a function of a leak that is creating the sound, but a function of air and water pressure acting together to making a wave of sound that can cause your pipes to shutter. So by securing you plumbing pipes where it is possible can help you with getting rid of this noise.
I once had a situation where a customer contacted me about this strange rattling noise in their wall. By the sound of it it seems like it would be a case of air hammering. The customer explained that they had turned off the water at the main water main and opened up all of the water valves in their home trying to draw off the air pressure, but they said that after numerous attempts they sound just would not go away. The person explained that the sound so much annoyed them that they begin tearing out the wall. He explained that he was not quite sure where the sound came from because of the sound was somewhat muffled by he said he was sure it was something due to his plumbing pipes and too much air in his system. So he repeatedly made the sound occur by turning on his bathtub water and when he felt like he had a reasonably close estimate on where the sound was coming from, he started making holes in his wall where he thought the pipes were. He said his plan was to secure the pipes because he believed they were just knocking on wood and that was his plan. By the time I got over to his place his wall was full of holes and the owner had a look of resignation as he could not even find his own pipes. Sadly after investigating the situation more closely, I had to explain to him that what he was hearing was not air hammering but just the common noise of water running through his pipes. It turned out that his hearing was so acute, he mistook the sound.