Not sure about suggestions, other than what you are already doing. I do know people who have used a spray of some sort (diluted lemon juice) as a negative consequence, but I’m not sure how effective that really is.
I think it’s more excitement from anticipation than stress though. Most of the dogs I know who vocalize on the line do it because they are flying high as a kite with the anticipation of the retrieve. My guess is if you can de-link the quack from the launch so the relationship is no longer automatic, the behavior may stop, but vocalization like that is a tough one because the dogs often aren’t aware that they are doing it.