What causes the whistling sound when I...

The whistling occurs when there is a restriction of water flow in the pipes. This phenomenon can also happen with cold water. The cause or the restriction may depend on the type of plumbing. If it is copper piping, this may be a sign that your plumbing is too small for your usage. If it is galvanized steel, it may indicate a lime buildup somewhere, which can reduce the opening through which the water flows and cause the high-pitched squeal.