Friends:
this morning I chanced to listen to the birds here on
Tiger Mountain near Issaquah after an extended trip
away. One of calls I heard was new to me and somewhat
odd. Instead of the more typical chittering/trilling
or squaking that seems common here to most birds, what
I heard were sustained, nearly single note calls that
lasted for one to two seconds repeating perhaps every
5 to 10 seconds, with each call repeated at one of
three distinct pitches. Occasionally, I heard the
peeep trail off slightly in pitch just toward the end.
So:

Peeeeeeeeep (middle pitched) for 2 seconds

5-ish second pause

Peeeeeeeeep (lower pitched) for 2 seconds

5-ish second pause

Peeeeeeeeep (higher pitched) for 2 seconds

I couldn't see the bird at all but it was well up into
the tall trees that so thickly cover the mountain
here.

The calling went on for several hours.

Any thoughts on which one of our feathered friends
that might be?

Many thanks in advance
dtr.