I had a long lasting problem with my MacBook Air 13'' for almost two months. The bottom cover was making some strange sounds when notebook was moved or was working with it on my laps. So I decided to contact Apple support where they wanted to resolved it under warranty but it would stay 30 days with service. Urgent fix within two days would cost approx $20. Based on some apple forums I found that the issue is with screws on the bottom cover.
So I decided to surf the web ans search for more stable solution like to visit service more often and I found that special screwdriver that can be used to tighten the screws of bottom cover. All took me 1 minute and all is fixed now.
Go ahead and buy the special 5-point screw driver. Fix of this issue is fairly easy and quick.