Users found a certain hissing noise to come from the back of the phone when they were using the device. Users found the noise to be annoying and some felt that it was outright embarrassing to have their phone produce a hissing sound when they used the device in a public place. The noise usually gets louder when the iPhone is being used for a heavy weight application or for a CPU intensive game. This is how people were able to identify that the source of the sound is the processor.
Give it some time and the problem may fade away on its own. This hissing sound is commonly known as ‘coil whine’ in PCs. When you begin using your iPhone regularly, the problem automatically subsides when the internal parts of the phone are lubricated. But if the problem continues then you may consider visiting the Apple Store for help.