Its hard to say without hearing it but it sounds like it may be a hard drive tick. If this is the case then back up your data immediately because its only a matter of time before the drive dies. What you can do is take the cover off of the computer and then turn it on and listen to where the sound is coming from. You can use something like a paper towel roll and put it up to your ear to try and isolate the sound.
If it turns out to be the drive then you will need to backup your data and get ready to do a Windows reinstall along with all of your programs. You may be able to have the drive cloned so you can keep everything as is and install the new cloned drive in your system and boot up as normal. You can install some hard drive testing utilities that may give you a better idea of what is going on with your drive. To find out what brand of hard drive you have go into Device Manager and find out. Then install the appropriate tool for your drive.