If your lens when being used at 2.8 has low contrast and you try to reduce that effect by printing on a higher contrast paper you could make the grain more distinct and prominent thru increasing local contrast. Is the negative shot at 2.8 denser than the negatives stopped down? Are both the negatives equally contrasty? If you have a grain focuser that is high powered say 25x is the grain in vieiwing the negatives at 2.8 and 16 similar? Are both negatives taken of the same subject and light conditions so that you are comparing the same areas on the print?