training people, guarding behaviour, is it a major problem?

[ditavonteesed]

If a dog guards say the car when people approach, apparently sounds like he is going to tear you to pieces but is fine once the door is opened, how would you go about training this behaviour out?
Also if like this in the car is the dog also likely to be like this in the house, would it be if someone knocked on the door or is it more likely to be every time someone walks past the window?

we live in a terrace and the neighbour (plus postman, plus paprboy, plus about a million other people) have access across our garden, would this be likely to be a major problem in a dog who has this guarding behaviour.
The rescue think that he has been taught this behaviour, would that make it easier to untrain?

If the dog is trained to 'speak' then it also needs to be trained to be quiet. Will take time and patience - same as everything for training, but it can be done.

My old GSD used to bark if someone walked past the car or if a motorbike sat behind us in traffic, but when told to shurrup he did. He also used to bark if someone came to the door or postie put post in the outside mailbox, but that was his job really. To alert - not to guard. There is a difference.

My 2 current GSDs are not woofy dogs at all and infact I have missed people coming to the door because they didnt bark, so I have taught them to make a noise when someone comes to the door, but to be quiet when I say so.