Bark bark howl usually means they noticed you as human or knew something was wrong, in my expirience. They may have also thought you were a bigger badder coyote in your own yard type deal. It kind of depends on your calling sound. When this happens to me I try to move un-noticed to the coyote because you know their location. I travel to a new location leaving my call on vole squeeks in my original location and get down wind a hundred or more yards, kind of a spot and stock hunt. If that effort does not work I then try a ki-yi or dying rabbit.