I have recently noticed a rather irritating hissing noise coming from my left internal speaker. It only seems to occur when the MacBook plays a sound or some music. When the sound/music has finished, the hissing continues for about 30 seconds and then stops. It also stops if you plug something into the headphone socket, and continues as soon as you remove it. I'm sure it has only started happening recently. It is not the fan noise either. Any ideas?