recently my household bought 2 macbooks, it were both great. However, we notice that when we press the on button to turn on the both macbooks, we could hear sound like the cd rom's mechanism moving from it place (or it could be something else, I don't know). The problem is one of them sounds louder and longer while the other one sounds softer and shorter. My question are: Is this normal? Do anyone of you have the sound like ours? If it is not normal, what is the problem and how to fix it? Thanks