When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Not that it pertains to this thread but here you go.
3GS's have an issue with rear calipers locking up. You are probably hearing a pad that is hanging.
Have your rear brakes checked. Sliders lubed or replaced.