I had these noises when I throttled (closed down) the valves that fed the toilets. The valves are sticking out of the wall behind the toilet. The seats vibrate when water passes them. When they were backed out all the way, the noise stopped.
A column of water is like a steel rod, water doesn't compress much. So when a vibration starts at one point it is carried throughout the water column and the pipe. (That's why whales can communicate across long distances.)
If it is not those particular valves, it is a valve or restriction somewhere along the line - possibly the tank valves themselves.
To see if it is the tank valve - take the tank top off - flush - as the tank fills , lift the arm gently and quickly to stop the flow of water. If the noise doesn't occur - the tank valve is the probable cause and you can replace it. The noise would occur as the valve throttles down the flow of water as the float nears the top of the tank.