Some of my runners (6 weeks) make a quiet “huurp hurrp” chatter while some still whistle and peep. But that sounded like my welsh harlequin when she first started trying to quack (she’s the same age and already quacking big time)
Looks like they are overheated, what's the temp in your brooder, they need to have plenty of space to move away from the heat source and find their sweet spot.