I heard one calling yesterday from a brushy thicket near a marsh. First I thought Catbird, but not really a Catbird song. Then I thought Brown Thrasher, but the habitat was wrong and the song was too harsh.
It was a Chat, and I did get a fleeting glimpse of the yellow too.
They are not uncommon but usually only found by their voice during breeding season. Found breeding all over the US especially in brushy thickets in the sun.
They are thought to be an odd form of warbler. Song and other details at All About Birds.
Have you seen one?