I have an old porcelin toilet, circa 1950's, that is in excellent shape and is 'ming green', with a matching sink, also in excellent condition.

Lately, when the toilet is flushed, it makes a sound like a siren going off, and doesn't stop until the tank is filled back up. The sound is very loud, and increases in volume until the tank is full.

I replaced the flapper some months ago, and cannot see anything obvious as to what could be happening to cause this. The water valve is on all the way. It almost sounds like it is having to fight to fill the tank back up.

Any ideas?