Help please, Dog barks when left at home, what can I do?

[CeeYouNextTuesday]

Corky is always out with me, or at home with someone, bar one day a week, when we all work. Mum lives next door and pops in to let him out for a wee, and will occasionally take him for a walk. We leave the TV on, he has food and water and I hide treats for him to hunt out. The problem is, as soon as we leave, he starts to bark and howl and poor Mum is being driven to distraction. Its often triggered by cars pulling in to the garage next door, lorries passing, and the phone ringing, and can go on for AAGES! This is learned behaviour, his mother showed him how before she passed away, and he only does it at home. I wouldnt say that this is necessarily seperation anxiety, (his mother did have trouble with this,) Id say more that he is guarding, and that he also does it because its what he has learned to do. What can I do?