Why would keypad on macbook makes a "cracking" sound when clicked

I have a macbook, almost 5 years old, that has a 2-part integral trackpad where you scroll around with the upper portion and click on the lower portion. The clicking part has become harder to press than it was originally and makes a rather loud "cracking" sound. I have another similar macbook and the clicking of that part makes only a slight "tap" sound, so obviously something has gone wrong with the other macbook. It is quite annoying to hear what sounds almost like something should be breaking when I click on the trackpad. Is this something that I can fix? Is it something that the Apple technicians can fix at moderate cost? Or is it something that is so costly that I may as well get use to it?