steering wheel won't lock, and sounds like it's loose

Ok well, just got this 73 Duster, and over the last couple of drives around town it has started to develop a clattering in the steering column. It has an aftermarket grant wheel, btw, but otherwise is a column shift. It doesn't lock when it normally would, with the key in the off position. I can feel it wanting to grab the lock plate, but it doesn't. The real problem, is that while I'm driving, I can hear all kinds of clattering when turning the wheel. I'm a little worried that something might grab or stop it from turning, so I've quite driving it until I fix it. I'm assuming something is loose or broken in the mechanism that makes the steering lock work. Any ideas what I might be getting in to?