At 3 weeks of age, I was hearing quack-like noises from my girls. The boy continued shrill peeping for weeks, probably until he was mostly feathered at about 6 weeks. Then he started making noises like a raspy "woot woot". He sounded like he was trying to mimic a train horn. 
Here's a video of mine around 4 weeks to compare the sounds. While at this stage, my females weren't quacking all the time, they do make noticeably different sounds.
Here's a video of mine around 4 weeks to compare the sounds. While at this stage, my females weren't quacking all the time, they do make noticeably different sounds.
