I recently purchased and installed Leopard on my iMac G5 because I was running on an older operating system (Tiger? 10.3 I think...). I upgraded so that my computer would work with my new iPod touch. However, now my computer is making strange noises periodically, and if left on for several hours, the computer makes horrible noises and eventually shuts off. Any suggestions? My iMac is a few years old. Thanks in advance for any suggestions.