A Small Bird With a Huge Voice

Good morning,

Can anyone identify this bird?  It appeared to look like a female cardinal, but the coloring is gray, not muted red. 

I only noticed the little bird high in one of our red bud trees because of all of the noise she was making.  She was extremely distressed that I had walked into the front yard.

When I moved away from the tree, she became quiet.  I am guessing she has a nest in there.

All the best,
Lois