Neighbours of ours took on a Dalmation which was unwanted by it's previous owner - it looked around 6 months when it arrived .
On occasions in the past i have heard it howling for an hour or so but not on a regular basis  .
Today it was at it for about 5 hours , and it rained for most of the time ( perhaps they took the kids on a day out and thought the dog would be better in the garden ) . I can't see in the garden so don't know if it's got a shelter/water/food .   I actually saw the owner walking it this evening and i do think they care for it , however the dog is obviously distressed to howl .
So , what should i do ? If it is well cared for the RSPCA won't be interested and as it doesn't regularly howl neither will Enviromental Health . The howling doesn't bother me that much but i am concerned that the dog is distressed when left in the garden. I am also too chicken to confront them !

Have a friendly word with them- they may not know what the dog is up to.  I friend of mine has the sweetest little dog, so well trained and so quiet they weren't even sure he could bark.  He went to work with my friend so very rarely left.  One day they got a knock on the door to say did they know when they left the dog barked and howled from when the car left to when it pulled back in the drive.  At first they couldn't believe it so they left a tape recording while they popped out.  Constant howling.  A bit of training and now hes quiet as a mouse while they are out.  They also brought the neighbour a bottle of wine as a thank you as they were unaware that their baby was under stress.  Happy ending all round.
Dont go in guns blazing, just let them know that their puppy is showing signs of separation anxiety.  Very normal for a rescue dog.  If you say they care for the pup they will want to know as they would want him to be happy.  Or you would hope!