This is a common problem but it can be hard to find in a building with more than a few toilets. The hydraulic properties of water cause the vibration to carry through the entire waterline system of the building. Many people say this is caused by air in the waterlines. They are incorrect. If there were air in the lines, the effect of the air would be to eliminate the hydraulic properties of the water. The problem here is the diaphragm in the fill-valve (ballcock). Even if the diaphragm looks good, it is not. It can not be cleaned or repaired in any way. Replace the fill-valve diaphragm and adjust the water level in the tank and the problem will go away. The only way to identify the bad ballcock in a building full of toilets is to shut off half of the toilets at a time near the sound until you are down to one. This can take some time, but knowing what you are looking for helps a lot. All the tenants of the property will band together and throw you a parade when they see how clever you are.