Peppy howling

In June 2003 we brought home a stray puppy of mixed breed that we
found on a
reservation.
He is very friendly and playful. We took him to the vet for a checkup
to
make sure he wasn't carrying anything.
The only problem the veterinarian found was that he was born with
canine hip
dysplasia.
As a result, he hops with his hind legs... but does not appear to be
in any pain. As I said, he is very playful.
We live in a small rural area town approximately 1 klm (3/4mile) from
a
train track. I'm pleased to report that practically everyone on our
street
has dogs. So thankfully, the complaints are extremely few and far
between.
The reason I mention the complaint thing however, is because Peppy
howls incessantly whenever a train lays on the horn going thorough our
town. He is the only dog in the neighborhood that does this...day or
night. I'm worried that maybe the horn is hurting his ears. Calling
Peppy, whenever a train blares its horn, makes him stop. But I can't
be outside with him all the time as he is an outdoor dog. We also have
two other dogs outside that we saved so he is not lonely by any means.
Does anyone know why he is doing this and recommend how to make him
stop? If he has coyote in him, would this make him howl? Thanks for
any help.