Whistle pigs, as they are known as in some circles, are rather timid critters. They don't like open ground much at all for fear of being nabbed by a predator. If there is any high grass or other cover near the hole you found, cut it down, if at all possible. You can use their fear of predation by installing things in the area that move - especially some that move eccentrically with just the slightest breeze. The cheap, aluminum pie pans can be of some use - especially if they are cut into smaller wedges. Don't bunch them together, though. Spread them randomly throughout the area. Groundhogs are very afraid of hawks. If you can, paint one or two halves of the pie pans a dark color and suspend them high overhead on string or fishing line.