Chances are the pressure won't change if you don't open a faucet to allow the pressure to drop. Let some water out then recheck pressure reading. If it was only the one toilet, that caused the noise, then I would blame the Fill Valve and suggest you replace it. But I believe it would be too much of a coincidence to have all 3 fill valves go bad. The next thought would be if the 3 appliances, causing the problem, were fed by a common supply, that was not also feeding other devices. I doubt that is the case also. Just for the hell of it, turn on the cold water in you bathroom sink while the noise is happening to see if it has an impact on the sound. In other words we're simply increasing the velocity to see if there's an impact. EDIT: AS far as, "pressure is jumping from 40 to 50 psi.", while toilet is running. If it's while you're getting the noise, it likely the velocity changes/surge pressures. That would stand to reason.