What is causing a knocking sound when I apply the brakes? on 1994 GMC Safari

The knocking sound only seem to be heard from under the back of my van whenever I slow down and come to a complete stop. There is no sound after I stop nor after I resume driving. Sometimes the knock is slow and and rapid. The sound is frequent.

Asked by for the 1994 GMC Safari
brake drums out of round as stated
1 more answer
Warped brake drum or rotor comes to my mind.