I have a three year old border collie/ pitbull cross that has a growling problem. He growls whenever someone walks in a door, including me. Normally he stops growling right away once he realizes who it is but other times he continues growling for minutes after, it is starting to cause issues between my step dad and I and he has to be licked up when people come over because they are scared of him. He is the gentlest dog ever, and I trust that he would never do anything to anybody. He LOVES kids, it's just when someone comes through a door. I'm really struggling with this behavior and I've tried multiple different techniques and I've also worked with a trainer. Any recommendations?