1 week old duck making a whistling sound
Could he have some food dried/stuck in his nares? This sometimes happens if they aren't clearing out their nares properly. Do they have water deep enough to dunk their heads and blow out their nares in? They should have that all the time, plus bath time every day to clean themselves off and be able to blow their nares out really good.
I have a 3 week old Pekin that does the same thing. Mine acts fine, has deep drinking water and a bath every night to allow him/her to clean out its nares but still does it. I'm new but from what I've read on here it seems to be common with Pekin's. I'm not much help, sorry but maybe someone else will chime in. I've read of people suggesting cleaning out the nares with a toothpick (mine looks like maybe there's a partial blockage) but I haven't tried it yet because I'm afraid of doing some damage.