There is surely a better description for the loud sound than "big rig". Anyway...
My best guess is a spring/part. A return wasn't attached properly or simply broke and is now laying in the bottom of the drum or a retainer/cap on a hold down spring failed.
In any case you'll need to pull the drums and have a look. There are conditions like loose parts that will lock down a wheel when you least expect it. I had a 71 Gremlin go nearly 180 degrees around once because the hook end of a spring broke.
