I have experienced a couple of different situations like this, one was simple and inexpensive, the other was rather expensive. The expensive problem I experienced was having to replace/repair the axle shafts as Benzmac suggested. The other was very simple, someone had installed brake dust covers on a car I purchased, one of the covers had a slight imperfection,(a bent place inward toward the brake rotor). Every time the wheel would turn, this ding would hit the brake caliper, this would make a ticking sound. The sound was not noticable at higher speeds, but was very annoying at low speeds. I hope your problem is this simple.