So we have a passerine with a brownish back and a bright yellow belly. Taking into account the bill and general shape and posture, we can say it’s a flycatcher. The brown color and yellow belly lead us to genus Myiarchus.
Among the Myiarchus, Massachusetts has Great Crested as a common breeder and Ash-throated as a rare (but basically annual late fall vagrant). Obviously, my wishful thinking comment was about Ash-throated but in this case, the belly is much too bright. The edges of the wings are also much stronger than I’d expect on an Ash-throated. And the vegetation is a bit too green for a late fall vagrant, although there is an August record at Rock Meadow in Belmont.
I took this picture of a Great Crested Flycatcher at Prospect Hill in May 2006. They are likely breeders at Prospect Hill, the Paine Estate, Met State, and probably any other wooded areas. As far as I know, Ash-throated has not yet been found in Waltham, but I’m sure one will one of these years.