[87 GT MUSTANG]

ok i need some help on this one, i notice a knocking or clicking sound coming from the *** end of my 87 mustang when i turn,back up or put it in gear sometimes, i was under my car yesterday and noticed a leak around my fuel lines and brake lines just above my rear differential but those lines are fine so i am wondering if maybe it is coming from my differential. any help would be great, she is going in the garage tommorow but i just wanna know if maybe anyone else has had simalr problems.