you have to try to hear where the sound is coming from. Does the sound go away if you turn off the monitor? if so then its the monitor. PSU's are commonly the cause of high pitch noise when they have some kind of electrical resonance going on. Its normally not going to cause any major problems, but can be a sign of a failure. If you have ever opened up a PSU you will have seen a white glue type substance between components. this is to stop them resonating and causing noise.