The sirens you're hearing in East Boston this morning are vehicles racing toward the edge of Logan Airport, as an abandoned vehicle with a suspicious device inside was spotted on or just outside a rent-a-car company. Some reports say that some residents have been evacuated. There's a live video feed here on Fox News.
Update: No explosive devices were found in the abandoned vehicle. Residents have been allowed to return to their homes.
Update II (Aug. 17): Car belonged to college student.