The bill size, overall shape (profile), faint wing bars, faint eye line and je ne sais quoimakes me think of juvenile Pine warbler. Oh my, I just realized that you made me think.Nice move, you tricked me into thinking.
I'm pretty sure it's an immature female Pine Warbler, they can be drab even in May, see the picture taken May 11 in the link below.https://www.allaboutbirds.org/guide/Pine_Warbler/id/Thomas