Most any kind of predator sound will make them pop up for a look. The hawk whistle is a good idea. Actually, they're about like Turkeys, in that any sudden, sharp sound will make them respond. The sound of a tractor will get them every time, or a slamming door, or even a shot in the air will get a rise so you can locate them.
Don't laugh at the tractor sound. If a person was serious enough, and energetic enough, he could carry an E-caller with a tractor recording and locate about every groundhog in the area.