My guy does this. It cracks me up and he is in perfect health so I am sure it is just him being a grunty old man. When he rolls, he does it too, starts groaning and grunting before he's even on the ground. All the little girls at the barn LOVE for me to let him roll and they all squeal with laughter.
My boy is noooisy and always has been. He some times hums when we warm up, sounds nothing like a pig, it literally sounds like someone humming. When he rolls it freaks people out because he moans, groans and grunts all the way threw his rolling...then he will lay in the dirt and rub his face in it and hum/moan some more. It has scared people because they thought he was hurt ;) When he plays he is noisy!
He's in tip top shape. I even asked the vet to check his air ways and she said he looked fine and it must just be him.
Vida is a grunter too. She grunts as she comes to her feed dish. Kind of like she is telling everyone "mine....mine ....mine" she does it when she rolls as well.
Some say its when they are in pain but I think some do it out of pleasure too.