Did you remove all the egg shells they can have nasty inside when they hatch and can smell.
How much progress has this duckling made?
since I don't use a bator I don't know exactly where the smell is coming from hopefully @New duck mommy 2021 will be here soon.
If you have a mirror that is safe put it in with the duckling so it can see itself and a stuffed animal without strings.
Congrats on the duckling :love