AFter flushing the toilet, there is a howling sound. If I turn the water on when it starts the noise stops.
This is only occurring in one of the 3 bathrooms.

Excessive water pressure, or the supply valve at the wall is not fully open.

It could be a lose washer in the angle valve, dirt or bad rubber washer in the fill valve.

1. The next time the noise starts start to back off the angle stop located under the tank counterclockwise until the noise stops, if it does then you have a lose washer inside the angle valve.

If you have a ballcock type fill valve do the following

2. Shut the water off to the tank at the angle stop. Take the lid off and remove the three screws from the top of the ballcock and pull up the float ball, rod and plunger assembly. Pry out the rubber washer and turn it over. Before you reassemble the ball cock put.

Please remove the toilet tank lid check inside and look which type fill valve do you, see the image below, Thanks.

John