Just wanted to post my own experience with this phenomenon. It absolutely is caused by loose screws on the bottom of the laptop. However, in my case, tightening of ALL of the screws was impossible as one of the screw holes on the bottom of the case came 'stripped' (the one on the bottom left of the left side hand rest). So, unfortunately, this will not go away for me. I can tighten all the other screws (and have tried mixing and matching), but i cannot tighten that location. It leads to creaking of the hand rest and the base feels 'loose' most of the time. Im not posting because i think its all that huge of an issue (i.e. i dont plan on replacing my unit), i just wanted to post my own experience since id seen this topic discussed on here previously.