Kittens meowing a lot

My cat had kittens last weekend.. I noticed that when they drink from her, they sound like they "complain". They are quite vocal kittens, but when mom leaves the nest they are nearly completely silent (not always, sometimes they keep complaining but a lot less loud). Why do they meow a lot when mom is around, and complain when drinking (like humming sound)? They also seem a little restless. They are growing well (10g+ a day).

Our kittens make a humming sound when they feed, at first I thought it was the sound of suckling but now it almost sounds like Purring! Could just be that they are very content