|You may be hearing a warning that your CPU has reached a preset temperature that is approaching the shut down temperature. |
This can be caused by a build up of dust/dirt inside the case. You can verify the temperatures by installing a utility to monitor them. Download and install HWMonitor from the link below. Post the temperatures after you start hearing the beeps.
The best method to clean out the case is to use either a compressor or compressed canned air. Canned air is sold at office supply stores.