I am not a hatcher, wish I could give you some wise words from experience, but I always seek the help of the others who have posted on this thread - perhaps you could try a PM?
I am not a hatcher, but have read that the mallard types take about 28 days and that could be as early as 26 or closer to 30 days, depending on conditions in the incubator.
So, with this hybrid, I'd say it does not seem odd that you are seeing some rocking.
@madamiec , @casportpony...