Theres a water main break in my area and my upstairs toilet pipes are whistling. What does that mean?
Sounds like air has gotten into the system. When the water comes back on, expect there to be some loud bangs as the air is pushed out of the pipes by the returning water. It's enough to startle you if you don't expect it.
That would be air in the system. You should partially open ALL the taps and let water run for several minutes after the water comes back on to clear the lines.
Due to the break and water leaving the lines its drawing in air, creating a whistle. You may hear it again when the lines fill.
It is called suction.