Grunting rat?

Question
Recently I have noticed that one of my male rats makes this sort of low almost inaudible grunting/oinking noise when he is running around with his two other cage mates during play time. First  I noticed it when I introduced their new play area with cardboard hideouts, dig box and wading pool so I though it was excitement. However just a while ago I had all three out upon my bed running about and I noticed that my chubbier of the three was running about making these oinking noises, I had played a bit with him and he was jumping about and running around playfully and then started again when I went to go wrestle with him. Also he likes to nip, kind of like when they groom affectionately but sometimes he randomly nips my ears and cheek and between my fingers. So now I am wondering if this grunting noise is a sound of aggression or if its a dominance thing.
Side note: Two of the rats are from the same litter, the questionably aggressive one and the second biggest one, and the third is the smallest and from a different litter.

Answer
Your boy is not aggressive.  He nips at those places just to taste and explore. Even a slightly aggressive rat will show aggression  usually toward their cagemates first.

The grunting your hearing is the start of mycoplasmosis. Usually the chubbier rats will be the first to make this sound. When he sneezes, does it clear up for a while?

He needs a vet that is familiar with treating mycoplasmosis pulmonis. He should be on baytril for 30 full days, no exceptions.  

What type of litter do you use and what kind of food does he eat daily?