Reader John found the answer in an article in the Worcester Telegram. Turns out it was part of a military training exercise:
The helicopters, which were black and had only a couple of small lights on their underside, had an eerie ability to be heard but not seen. At 10 o’clock, for instance, the church bell at Wesley United Methodist tolled just as a nearly invisible but thunderous handful of helicopters rose from Lincoln Square and flew by.
Maj. Potter said units from across the state were doing military operations and urban terrain training. She said the exercise, which started about 9 p.m., was coordinated with the appropriate city and county officials and property owners.
“All safety precautions have been taken to prevent unnecessary risks to either the participants or any residents or their property,” Maj. Potter said. “We apologize for any inconvenience or unforeseen disturbance.”
A similar training event rattled Southborough during the first week of August last year, so perhaps this is becoming a yearly event?