Warning: Opening your computer can be dangerous if you don't know what you're doing. Make sure to unplug your system, and ground yourself before touching anything inside of it. If you don't feel comfortable making these repairs yourself, then hire a professional.
Below are possible sources of noises from your computer:
- Hard Drive: If you start your computer, and right away you start to hear some strange sounds (such as high-pitch whirling or clicking, listen to the examples on this page) from your hard drive. This is generally an indication that it has failed or is about to fail. If you can access the data on your drive, I would back it up to another location (such as an external hard drive) as soon as possible.
- Power Supply, System, or CPU fans: If you start to hear strange noises from your system fans, this could be an indication that your fan is dirty or has bad ball bearings. If this happens turn off your system and clean the the fans or replace them as necessary. If you want to see what happens when your CPU loses cooling, check out the following video. If the fan that is giving you problems is in your power supply, do not try to replace it. Just replace the whole power supply.