That sound is your hard drive (unless there's something wrong with your fan, in which case your machine is probably on the verge of death). If you don't keep backups, start.
Constant hard drive access usually means paging -- check the memory usage in Activity Monitor. CS3 is a memory hog and 2GB is pretty low for photo, graphics or video work these days, so increasing your RAM will probably help. Mine was usable with 2.5 GB, but I was constantly checking what else I had running when using Photoshop. It's been smooth sailing since upgrading to 4GB.