I recently did some plumbing work and drained my water lines to complete the job. When I turned the water back on, I noticed the sound of the water running through the cold water pipes when I flush the main floor toilet is extra loud. Cold water use at any other fixture in the house does not create this loud, rushing water sound from the pipes. I replaced the tank fixtures and added an arrestor to the cold water supply thinking this would help. It didn’t. No knocking like water hammer; just loud rushing water though the pipes when the toilet tank is filling.
As per my knowledge, faulty toilet fill valve will cause resonance sounds like a foghorn. Hence, I would recommend to try closing the supply valves to all the toilets. If the noise disappears, open them one-by-one till the noise starts again.