whistling after 11pm?

Hi, we have a bird (i think) that emits a whistle lasting around 2 seconds give or take. It is one single note that does not seem to waiver. it is like a human's whistle with just a little too much air.

At the end it sounds almost as if it runs out of breath.

We live on the North Olympic Peninsula, in Washington State. We have not heard it before, we have been here 20 years.

by searching the internet i have determined that it is not a saw-whet owl, or barred owl. Further i have also determined it is not a fox, though the sound is similar to the fox yell at this address http://www.geocities.com/RainForest/4707/FoxSounds

That yell sounds very canine and what we hear doesn't sound at all canine.

Also between some whistles which happen sometimes every 15-20 seconds or more, it will move from one area to another.

The longest being about 80 linear ft.

There is no noise during its moves no trees rustling, no sounds. you know it has moved when it whistles again from a new perch, this is why im sure its a flying creature.

please help identify this night guest

Sam