The whistling sound coming from water pipes can tread the line between soothing and alarming. This sound is due to high water pressure, and is a sign that something bad is about to happen to the plumbing system of your property.
When whistling can be heard from a toilet fill valve, tap, or somewhere within the wall, take it as a sign of water flowing. When the sounds are coming from a valve or tap, it is a sign that the washer or gasket has worn out, and the sound produced is what happens when water passes that part. If the sound is coming from the pipes, it is due to high water turbulence, and reasons for this are:
The Pressure is On
The problems mentioned above have grave consequences. For instance, mineral build-up can loosen and block the pipe, air bubbles can prevent the flow of water, and improperly installed fittings causes whistling and vibrations, which weaken joints and cause leaks. High pressure is the worst of these problems, though, since it causes joint failure and leaks in several areas. If the leak is behind walls, it cannot be found until the walls are severely damaged.
To find out if water pressure is high, you can do a simple test. This test will involve a pressure gauge, which you should screw on a threaded tap (preferably one located outdoors or the laundry area). Once you have done so, turn the tap on and take a look at the needle. The water pressure must not exceed 60 PSI. If you own a well, the gauge must stay connected until the pump will cycle on and finishes pressurising the system.
There are two ways to reduce pressure:
Purging the Air
When air enters the water pipes, it goes up the highest point (at upstairs bathrooms or walls by those bathrooms) and will stay there to create a bubble. To purge air, you should first switch on each tap in the bathroom while flushing the toilet to increase water flow. This increase in water flow should push all air out through one of the taps.
Call a plumber if the whistling is from old steel pipes or if the whistling sound is from a specific fitting. The former is because of corrosion within the pipes and it cannot be fixed any other way. As for the latter, rerouting the pipes is the only way it can be sorted out.