Honeywell zone valves often stop clunking when one of the two return springs in the valve head under the sheetmetal cover are removed.
“I removed one of the springs, like he suggested, it took two seconds. The hammer was instantly eliminated. To be sure, I put the spring back on, the hammer came back, took it off again, hammer went away.”
If the head is silent now, just keep the old spring taped inside the cover. If the other spring breaks, it has a replacement – and the head will continue to work. The spring just returns the ball to position, more slowly now. The slower action is sometime enough to stop the slam.