I assume you've made at least a cursory look under there for loose things such as a shock mount, broken spring leaf, etc.
A couple of possibilities come to mind. First, the self-adjusters on these rear brakes are pretty notorious for failing, and if one brake has gone way off, it could knock when engaging. You should open them up anyway, just to inspect everything. Although the mileage is pretty low, the age is sufficient for rust problems to develop. Look especially at the adjuster cables, which can rust right off, and the "nails" that hold the shoes to the backing plate, which can rust, as can the springs between shoe and plate. Those can make noise when they go. The good news is that the hardware for these brakes is pretty cheap and easy to find. When you put them back together it's a good idea to do a manual adjustment. Even if the adjusters look all right, I don't trust them actually to do the job consistently.
When you get the drum off, check it carefully for cracks and damage. Unlikely, but a cracked drum can make weird noises. Again, although it probably isn't badly worn, it might have rust buildup at the edges, including the groove that goes over the edge of the backing plate, and this can also make noise when the brakes cause a little flexing of the drum. If it isn't too bad you can knock off the rust and clean the groove out a bit.
A final thing to check is the rear universal joint. If it is either loose or binding, it can make noise. I recently had a noise from mine, and found that it had a binding cap, which only revealed itself when I actually took the shaft out. But if you jack up both rear wheels, and put it in neutral (BIG HAIRY JACK STANDS, please! - we don't want to lose potential members!) you might hear a crunching or creaking when you turn the shaft by hand.