I was browsing a webpage with Safari when suddenly this really loud screeching sound came from my Macbook Air. I am not talking about the small clicks the HD seems to be doing once in a while. This was a very loud screeching sound, like someone cutting glass with a diamond. I didn't have any other programs on and my speakers were off. The sound lasted for about 2 seconds and got me very scared. I immediately checked the temperatures and fanspeeds but they were all normal (CPU around 45 celcius and fan about 2500 rpm). Now I'm afraid that it was my HD and it will fail on me sooner or later. Everything seems to be work normally now but I already sent all my important documents to another computer. Any suggestions on what to do next? Should I wait and see if it happens again later?