She has done it for a long time, usually when she eats or when she gets tired, and especially when she is doing both. I first noticed it when she was breastfeeding, so under one, and now she is still doing it at aged three and a bit. It's like a repetitive hum, over and over and over. Is this a bit weird or is it just a personality quirk?
My DS (3) makes a sort of mmmmm mmmmm mmmmm noise when he eats. He only does it when he likes what he's eating (like icecream!). He's done it since about 7 months. I think he was copying me saying "Yum" and it's just stuck.