I agree with Adrian. In the dark of morning and at dawn's early light, a hoot owl will work. After daybreak, a crow works better. Mid-day, I think a loud cutt probably brings the best gobble response, but there are obvious problems with using a turkey sound as a turkey locator, so I think it is better to stick with a crow call at midday as well. Another call I use for locating during the day is a pileated woodpecker. Sometimes this will work when a crow does not. I, too, use coyote howling in the evening/at dusk for roosting, with good response, "out west." I have heard turkeys shock gobble to honking geese flying overhead, thunder, town noon whistles/sirens, and car honking horns. Last year I even carried a compressed gas operated air horn like you get for small watercraft, but I never got around to actually using it, so I don't know how effective it might be. My suspicion is that the right tom will shock gobble for it, though.

If your locator calls have actually "never" produced a shock gobble that you have heard, I wonder if you are calling over the response gobble. If you are alone, always start with a short blast on your locator call and pause a moment before calling again. First, using the shorter blast helps to avoid creating a situation in which you have "blown the turkey out of the country" if he is nearby, and secondly, it gives you a chance to hear any "hot" immediate response. If you hunt with another, have the buddy get 10-15 yards from you before you blow on the locator call, so he/she can hear any distant response.