Regarding your buzzing sound--I know this is probably a long shot but perhaps it's a cooling fan coming on or something? Limewire X is quite CPU intensive--much more so than your normal web browser or email program, especially if you are configured to be an ultrapeer, are downloading many files, and/or are performing many searches. In theory, this could be causing your laptop to heat up much more than it normally does while using other programs, hence the cooling fan coming on.

A ".sit" file, as David91 said, is a stuffit archive file, very similar to ".zip" or ".rar" files on a PC. A .sit file can contain a single file or multiple files.

A ".dmg" file is an OS X Disk Copy disk image file. This is a single file that when double-clicked will be opened by Disk Copy and "mounted" on your desktop. The disk that is mounted is like most other disks in that you can explore it and store files on it, and depending on the type of image, read or write files to it. Most .dmg's you'll come across are read-only images and are meant only for you to copy files off of and not onto. Once you are "done" with the disk, select it in the Finder and hit Command+E to "eject" the ("virtual") disk.

".img" files are OS 9 disk image files that are basically the same as OS X .dmg files except that they can be used in both OS 9 and OS X.

Hope this helps....