Then I suggest you pull the cover off the side of the thing and figure out if it's a HDD or fan making the noise. Either one is something you want to address ASAP. Either because you have a closing window on backing up any data, or because you have a closing window before a bearing seizes up and something overheats.
My Mac Pro is beginning to make a regular and repeated sound. One of my hard drives is highlighted on the desk top. The mouse does not work correctly and I have to shut down to stop the sound. The problem is intermittent.