For weeks, residents living in the shadow of the former Fort McClellan have reported hearing a low, persistent hum. The sound, described by some as a "pleasant thrumming" and by others as "like a refrigerator having a good dream," has been a source of community speculation.

On Tuesday, workers from Alabama Power located the source.

The sound is emanating from a large, green, box-like electrical transformer located underground near the corner of what was once 1st Avenue and 10th Street. According to a statement from the utility, the transformer is operating well within normal parameters and poses no safety risk.

The hum, however, is unusual.

"We've run full diagnostics," said David Riley, a senior technician with Alabama Power who was on site Tuesday. "The voltage is stable, the temperature is optimal, and the structural integrity is sound. All our instruments tell us the unit is perfectly healthy."

When asked about the source of the unique auditory signature, Mr. Riley consulted his notes. "The best way we can describe it, based on the harmonic resonance and the frequency modulation, is that the transformer is... particularly content."

Further tests performed by the utility workers confirmed the initial assessment. The hum reportedly becomes slightly more pronounced when the sun is out and diminishes to a barely audible murmur during periods of heavy rain.

Alabama Power has stated that since the transformer is functioning perfectly and is not a danger to the public, they have no plans to replace or disturb it.

"If it's happy, it's happy," Mr. Riley said. "We're not in the business of fixing what isn't broken. We're just going to let it be."

The utility has placed a small, orange flag next to the patch of ground above the transformer and considers the matter closed.