mod05imprezawrx

hi i bought my 05 sti about a year ago and i started experimenting with its equipment. i found that when you adjust the center differential controler to fully locked it makes a kind of hard knocking sound while turning it sound like it comes from the front drive shafts or that area at least so im not sure if there is something wrong with my car or im just not using it properly. can someone explain to me exacly what each level or setting mean on the differential controller, i tried looking on the owners manual and it only says that when you turn the **** all the way to the front it's fully locked but it doesn't explain what fully locked mean, what does it do, or when it sould be used. and also i tried installing a autometer shift light (one that does not require a tach) and i could not find a cable with the signal for the green wire on the shift light. i tried getting it from the back of the gauge cluster but no luck. anybody know which cable is it or a better source.thanks