During peak gobbling, they will shock gobble at anything, at anytime.

I've found that during the middle of the night though, they tend to only gobble well at very loud noises---such as thunder---and do not respond well to owls or coyote yips.

Your best bet is +/- 1 hour sunrise & sunset. I prefer gobble tubes, crow & owl calls.

I don't like using the coyote howl, just because it's a natural predator...