Thank you all for the help on my mystery call report from yesterday.
Most replies I received fell squarely into two camps: a few for a Sora
and many for Great-crested Flycatcher. The maddening part of this,
however, was the total lack of movement in the scrub, trees, or reeds
when I was trying to find this mystery bird. Whatever it was had
stayed put for a half an hour until it finally fell silent. This
seemed to support a Sora deep in the reeds over the flycatcher, which
in all the times I've seen one of this species has rarely kept still.
But neither birds' calls rang quite true when I listened to them
online. So back to the bioretention pond I went today!

Bob Ringler wins the prize for correctly identifying the "mystery
bird." It was in fact...a groundhog.

I find it amazing that I had never heard a groundhog squeal before,
having lived Frederick County all my life, where they outnumber any
other living organism (not to mention that there's one living under my
front porch!). But right at the base of the tree stand from where this
squeal was emitted is a large den hole. For anyone else who has not
heard one and is interested in not making this same mistake, someone's
recorded their squeals at this link:

http://www.hoghaven.com/hear.html

This adds a new non-bird to the list of "animals impersonating birds"
that I have momentarily looked foolish chasing. Most recent roster
additions include chipmunks in leaves impersonating furry towhees,
field mice in a hollow oak impersonating nonexistent baby birds, and
as recently as tonight, a white-tailed deer impersonating a hawk (I
ran outside to see what had caused the flurry of finches fleeing my
feeders...the doe had nosed open my big hopper and was chowing down).

-- 
(sheepish) Alex Baish
Ijamsville, MD (home)
Baltimore, MD (work/school)