Thanks to all that replied to my original posting.

After skolding us and bringing their shots up to date, both of our cats
were givin a clean bill of health.  That was big bucks to me, givin the
amount of medical expenses I've had so far this year.

After  some discussion about many things she suggested that we open the
door from the house area into the garage.  This sounded like a bunch of
feline psychobabble to me, but it seems to have worked. The howling stopped  
that night and hasn't reoccured yet!!  

Seems like she prefers to have her box, water and food in the garage (where
it usually is during spring, summer and fall).  Her sister apparently
doesn't give a hoot(or meow).  

I think that the monsters that attack cats at night are for some reason not
able to get into the garage and this gives her a save place to hide when
they prowl the house at night.

Thanks again!!