Our son’s van transmission makes a clunk sound when it downshifts while slowing for a stoplight. Sign of a serious problem?
I would check the motor mounts especially the front and rear mounts which restrain the engine/transmission torque. Try shifting the tranmission in and out of Drive, Neutral, and Reverse while stopped and see if you can hear the clunk. Of course don’t stand in front of the vehicle. Look for excess engine/drive train rocking as a sign that the torque restraint bushing(s) has failed.
Hope this helps.