Why does my 3 week old make grunting sounds when pooping?

See below. Very normal. Your child is bearing down to poop, as we all do. Your child's face may also turn red when having a bowel movement.
Pooping. Children can make noises and their face can turn red when they are trying to poo.
Normal vocalizations. Infants make a variety of grunting & squeaking sounds on occasions with no specific agenda. They have no social conscience so they don't care if the sounds bother others. They need the assistance of their breathing muscle to help push poo along & often run a burp thru their pursed vocal cords. Enjoy the music & don't put adult reasons behind baby sounds.