For what it's worth on dogs with aggression issues, here's what the behaviorist had me do with a rescue and it worked within minutes:

OWN THE OBJECT! For Rusty it was the doorbell and vacuum (to start with). So I sent the kid out to ring the doorbell and had Rusty on a leash with a choke collar on. When the doorbell rang I instantly stood between him and the door and said sternly "no". If he moved forward he got a collar correct and repositioned. Any other sound or movement away and he got another no or snapping fingers to make him concentrate on me. When he moved away he got a "atta boy". When they start yawning they are ok with it. If their tongue is coming out like they're chewing on something they're nervous.

We did the same with the vacuum. I had to get in front of it and own it - I'm the alpha and the rescue needs to learn that. It provides them with security. It worked wonders! If I didn't catch it this 30 pound dog turned into Kujo and I got bit twice, so nip it in the bud! And please don't give the mail carrier treats. If the dog starts to growl, bark or attack and they're given a treat it will reenforce the behavior.

Hope that helps someone - I paid good money for that lesson!