Swish / Drum sound

So i am currently using my second macbook within a week. The first macbook's optical drive was broken and they sent me a replacement. I have always been a PC user and finally was smart enough to make the change to a mac. Randomly my macbook makes this weird swish/drum sound and its coming from the OS and not the hardware. It doesn't happen when i am playing music but if I'm just surfing the web or streaming a movie it randomly happens. I have gone through the preferences of every program i am running to see if there is some sound i need to turn off but can't find one. Is this sound normal? Should i be worried or am i just a paranoid new user?