As first noted in the comments of an iLounge news item, the audio defect found in Apple’s fourth-generation iPod and iPod Photo is apparently caused mostly by third-party headphones/earbuds that have a metal base around the headphone plug. When the metal base comes in contact with the metal case of the iPod, the static and hard drive accessing noises can be heard.
Several readers have provided temporary fixes for the issue. One reader suggests using a small piece of plastic wrap between the headphone plug and iPod, while another has had luck with a tiny teflon washer. Others have simply used scotch tape. Basically, any insulating material that can fit between (and around) the plug and the top of the iPod will work as long as it does not conduct electricity.