I agree with the others. Ours are taught to come to a whistle. I've also been known to yell "who's hungry!!" You can get them to come to most any sound, and that is taught through food, either by calling or whistling during dinner time, in addition to treat time.
Our horses were not intentionally taught to come to the sound of the 4 wheeler. We would load the food on the 4 wheeler then drive it out to the pasture and feed them. Now when ever they hear the 4 wheeler, (or lawn mower, or truck) they come a running. Sometimes it's funny, other times its a pain (like when you're trying to mow and they keep crowding you. lol)