People living in the vicinity of the Pennsylvania National Guard base at Fort Indiantown Gap are accustomed to the loud noises that sometimes accompany military training. But one sound lately has some residents stumped.

“Quite frequently, we hear a long loud ‘vroom’ coming from the general direction of the Indiantown Gap Military Reservation,” Brad Tipton, who lives on a hill overlooking Campbelltown, told LebTown in a recent email.

“The sound comes sometimes early morning, but more often in the evenings,” Tipton said. “It is not the booming of munitions or blasts when they have live fire exercises.”

He’s not alone. He said “many residents in the area” are also hearing the sound. A friend from northeastern Lebanon County thinks maybe the noise is “made by a gas (or other fuel) pipeline transfer station burning off excess gaseous pressure,” he suggested.

Read More: Business as usual at the Gap but atmospheric conditions cause artillery testing booms to travel further

In a follow-up email, Tipton explained that the “vrooms” are “heard sporadically, sometimes during early morning, most frequently during the evening, not during normal hours when we hear the Gap live fire, which is distinctly different.”

Sometimes in the evening, he added, “a second vroom will follow about a half hour to one hour after the first one, then quiet. These do not appear to be scheduled events, they are irregular.”

But, he stressed, the noise comes from the general direction of Gap, but he’s by no means certain of its place of origin.

“As I think back, when I have been outside during some of the events, the noise seems sourced slightly to the west of the Gap,” he wrote. “It is hard to determine strictly by ear at the distance we are discussing.”

Tipton said he and his neighbors would love to know the truth of the matter. The answer, however, is elusive.

The obvious starting point for an investigation is Fort Indiantown Gap. But a Gap officer wasn’t able to provide an answer, due to the lack of precise information about when the noise occurs, though he had a few guesses.

Read More: Increased security coming to Fort Indiantown Gap construction of two gates to start this spring

“It is hard to say for certain without specific dates and times,” Lt. Col. Keith Hickox, state public affairs officer said in response to a LebTown enquiry.

With more information, Hickox said, he could “cross reference with training that was actually happening at that time.”

Based on the data provided, Hickox said the source of the sound “seems to be a good distance away from our base so there is a possibility the sound is being generated elsewhere.”

If, however, the sound is truly coming from the base, he said, his assumption is Tipton “could be referring to the A-10 aircraft that occasionally use our range.”

“The A-10 has a 30mm weapon system that can make a sound much like hitting a highway rumble strip at a fast speed,” he explained. “But certain artillery pieces can also make a similar ‘vroom’ sound when they fire, unlike the ‘boom’ you hear from impact… None of these sounds are unusual in the vicinity of Fort Indiantown Gap, however weather conditions often affect how far those sounds travel. These changing weather conditions make the sounds more of a rarity as you get further from the base.”

Tipton said he will try to record the sounds in the future and perhaps better determine the direction of its source.

In the meantime, readers of LebTown who have heard the sound are invited to submit any details they have noted – dates and times of occurrence, frequency, volume, characteristics of the noise itself – as well as any theories they have about its source. Any new information on the mystery “vrooms” will be reported in a future article.

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Tom has been a professional journalist for nearly four decades. In his spare time, he plays fiddle with the Irish band Fire in the Glen, and he reviews music, books and movies for Rambles.NET. He lives with his wife, Michelle, and has four children: Vinnie, Molly, Annabelle and Wolf.