My appartment has squeaky pipes. When the neighbor flushes the toilet or takes a shower they make a loud, high-pitched squeal. It's never been an issue, and still isn't really, but yesterday, for the first time in my life, I thought, "WTF?" I cannot think of a scenario where water running through a metal tube could make that noise. What causes it. Is there any way to fix pipes like this, short of tearing up the walls and installing new ones?