TAPPING NOISE> The trackpad of our Macbooks are covered with a thin layer of smooth glass. What we see is in fact the metalic surface beneath the transparent glass surface. When we just touch the surface without putting any pressure, it usually makes no noise. The click noise comes only when we press down. My problem is that when I simply touch the surface around the bottom left hand side of my trackpad, it makes a small 'tap' sound as if the very thin glass surface is tapping onto the metallix surface beneath. Any ideas what this is and how to fix it? It's irritating the buzzards out of my nerves!