Whenever someone flushes a toilet (any of the three toilets) the pipes
make a very loud piercing/shrieking/whistling noise for about 10
seconds. Whenever someone runs (usually) hot water for the first time
that day from any of the four faucets, the pipes also make that
horrible noise. This just started happening a few weeks ago after my
dad was messing around with the water heater. I don't know what he was
doing but he kept turning the water (or main water) on and off several
times a day. He might have been cleaning something or doing something
with the water conditioner, I don't know. But shortly after this the
pipes started making the shrieking noise.

Anyone have any ideas on what the problem could be?