I've started to notice that there's a high pitch hum or ringing (sort of like a high pitch ringing in your ears sometimes) when the engine is running. And when I turn off the engine it's like turning off the vacuum cleaner (don't know if its related to the high pitch sound). Is there something I should be worried about? I've attached a photo of what region the high pitch sound comes from. The car is a 2001 Holden Astra sedan.
Sounds like you might have a vacuum leak. Check all the vacuum lines in the area. Some electronic components can also make a high-pitched whine when they start to fail, but the only ones I'm familiar with that do that are flyback transformers and high-current power converters, and I don't believe a car would have any of those. That said, it might be worthwhile to find out if the vehicle's ECU or other control units are in the area, and check to see if they're making any noise.