Sounds like it could be one of a couple of problems. The first could be a loose washer inside the hot faucet or valve you are turning on. When water runs across a loose washer or a washer with a torn piece, it causes a high pitch frequency (flapping of the gasket). This can sound like a whistle. Another problem can be a loose piece of calcium inside the faucet or valve body. This can vibrate as the water passes over it. In the first case, dissassemble the faucet and check the washer for any damage or for the screw to be loose. The second case, you will have to remove the valve and flush it backwards with water. Before you reassemble it though, flush the pipe out with water by attaching a hose thread adapter to the end of the pipe and affixing a hose to the adapter and run it to a floor drain or bath tub. Turn on the water to the house and try to blow it out. If you are attached to a 1/2 or larger pipe, you stand a good chance of getting the pieces of calcium out. The hard stuff on the inside of the pipe will remain though.