One thing you need to realize is there can be more than one fan inside your machine, such as: cpu fans, video card fans, system fans, and even fans inside power supplies.
Here are two tutorials on case maintenance: Computer Maintenance Guide
, and Cleaning the Interior of your PC
Especially check the part about fan rotation.
You can also go online to YouTube, search for fan, and or computer noises, and you will be able to hear what the different sounds, sound like.
Be specific, and use the name of your cpu, video card, system fans, and power supplies. If you built this PC yourself you will need the actual name of the component.
Is this, a brand name model or did you build this yourself?