I think its the disk drive because my macbook pro has a disk drive over there. try the eject button ( If there is even a disk drive)! Also try downloading https://www.macupdate.com/app/mac/23049/smcfancontrol It may look like a virus but its not i use it because my mac gets so hot and the fan is small. try to change the speed and see if it gets louder if so its a fan if not its a disk drive!