F-15s from the 104th Fighter Wing, Massachusetts Air National Guard, in flight in 2011. (Photo: Senior Master Sgt. Robert J. Sabonis)

What was that big boom around New Year’s? It depends which noise you’re wondering about on which day.

The “massive explosion” described by a Nextdoor user named Ariel Jacobs on Tuesday was most likely a flyover by F-15 aircrafts before the Bruins’ Winter Classic hockey game Monday at Fenway Park (in which the team beat the Pittsburgh Penguins, 2-1). The 2:20 p.m. flyover involved two planes from the 104th Fighter Wing of the Massachusetts Air National Guard.

“We did receive several questions/concerns,” said Jeremy Warnick, director of communications and media relations for Cambridge police. That’s putting it mildly compared with reactions online, which called the sound significantly louder than past flyovers.

“I’ve lived here for decades and I don’t remember anything that loud,” said a Twitter user identified as Tina Sparkle, while Rosalie Rippey said it was “sooooo loud” that “my husband and I were looking at each other like, it’s over. This is our final moment.” National security expert Juliette Kayyem identified the source without knowing about the scheduled event – “Whatever sound that was in the skies over Cambridge or Boston a minute ago, it was fast and low … some military plane, but no idea what or why,” and agreed that it was “crazy loud” compared with other flyovers.

But that sound had competition, and not just from New Year’s fireworks.

A transformer explosion led to a power outage Thursday around Cambridge and Prospect streets, and a “loud noise” and “possible explosion” was heard and reported at Cherry and Main streets just after 6 p.m. on New Year’s Day, Warnick said, citing police reports. The latter noise, from Sunday, was investigated “in the immediate area and neighborhood, but nothing of concern was located or observed. It’s possible it was fireworks.”

“I was about half a block from where it happened” said a Reddit user of the Sunday incident. “There was a loud bang and then a much larger boom with a flash of light, and smoke. My dog was freaking out, so I didn’t go out to check on it, but there wasn’t anything obvious left in the street where it happened.”

Though the flyover would have been the noise experienced across Greater Boston, the various noises were all reported widely – and their aftermaths: panicked pets, triggered car alarms and unhappy residents. “The whole building shook,” a Nextdoor user named Tara Denny said. “I just lost 10 years from that.”