Glenn Salmon

I have a 96 STI Import with a differential Indicator on the dash. It seems fine when the setting is on '0'. When I go up a notch the front wheels kind of knock, when I go forward slowly & reverse, I dare not to try further notches in case it knackers something expensive. Has any one had this kind of problem before. If so please help me in to finding whats wrong, as I cant see anything wrong underneath the car, i.e. drive shafts etc look ok.