2006 Subaru Impreza Owner's Manual - Page 217

7-20 Starting and operating gine. Take your vehicle to the nearest SUBARU dealer immediately to have the system repaired. Limited slip differential (LSD) (if equipped) The LSD provides optimum distribution of power according to the difference in revolutions between the right and left wheels that may be caused by certain driving conditions, thereby improving driving stability on snow-covered, muddy or other slippery roads or when high speed driving. If you use a temporary spare tire to replace a flat tire, be sure to use the original temporary spare tire stored in the vehicle. Using other sizes will adversely affect the LSD. NOTE (WRX-STI) The rear differential may emit a knocking sound when the vehicle is driven slowly with the steering wheel turned fully to either side. This sound occurs because of the design of the mechanical LSD. It does not indicate a problem. Never start the engine while a tire on one side is jacked up, as the vehicle may move. If one tire is spinning in mud, avoid continued spinning at high speed as this could adversely affect the LSD.