I have a 3 1/2 week old little boy and he not only grunts after a feeding, but when he is lightly sleeping. I am glad to know that I am not the only one - I was constantly worried that there was something very wrong. I could not sleep because of this.
My doc said partly due to the larynx not quite developed as well. I suppose we just have to wait to grow out of this. I did not realize that so many babies were noisy sleepers - it was quite shocking to me.
Hopefully soon I will learn to not worry quite so much and just wait it out.