They arrive late, under cover of darkness, pulling up to the Kennedy Center in their rented white Ford Fusion just as the last patrons are heading home. Six nights a week for six weeks in a row this fall, Richard Marchand and Daniel Fortin have been the center’s most regular visitors.

A security guard greets them at the door and ushers them to the Concert Hall, where, shortly after 11 p.m., the two quiet and unassuming French Canadians start their workday.

First, a coffee.

Then the night noises.

Brrrrroaaaemmmmmmmmmm. “Non!”

Pipes that make up the facade of the new organ. (Matt McClain/FOR THE WASHINGTON POST)

Dink, dink, dink.

Brrrooemmmmmm. “Non, non.”

Bbbbrrrruummmmmmm. “Oui, oui. Bon.”

Marchand is seated at the brand-new pipe organ console on the Concert Hall stage. The console has four keyboards, an enormous foot pedal system and more levers, switches and geegaws than you’d find in a 747 cockpit.

He presses on a key and holds the note. A giant burr slowly fills the empty hall. The floor trembles ever so slightly. Behind the stage and up a flight of stairs, Fortin stands amid a warren of 5,000 pipes, also brand-new, making minute tweaks until he and Marchand are satisfied with the resulting sound. Then on to the next pipe. And so on. Hour after hour, night after night.

This organ, this $2 million musical behemoth given to the National Symphony Orchestra by Kennedy Center Chairman David M. Rubenstein, and recently christened the Rubenstein Family Organ, is the building’s newest resident, and Marchand and Fortin are midwiving its delivery. They are tuning and preparing it — note by note, key by key, pipe by pipe.

Five thousand pipes.

Their deadline is its grand official debut, a free concert with the orchestra Tuesday.

For the NSO, the new organ is a godsend — even if God took his time sending it. Its predecessor, the Filene Organ, which arrived in 1972 and is the only organ the Kennedy Center had ever known, has been mostly silent for a couple of years. And before that, it had been a bit . . . unwieldy.

Near the end, damaged by water and age, the organ was the source of some embarrassment. It leaked bleets and blurts — and other sounds that shouldn’t be heard in public. It had no manners. It would “speak” when it wasn’t supposed to. In organ terms this is known as a cipher and the Filene had some doozies.

“It’s like a heckler interrupting a speech,” says the NSO’s organist, William Neil. “Over the years there were some massive ciphers that sabotaged concerts.”

Neil doesn’t want to pile on, however. The Filene had many good qualities, he insists. But the old organ was never a perfect fit for the NSO. Its maker, Aeolian-Skinner, was a great company but it was going through bankruptcy at the time it was building the Filene. The organ was not its finest creation.

“It had a bright, brilliant sound but it was no match for organs of the 19th and early 20th century in terms of symphonic power,” says Neil.

The previous organ never quite had the depth or oomph to fill the hall, satisfy the musicians or excite the audience. This new one, everyone says, has enough lung power to blow the Kennedy Center from its foundation.

The organ voicers

The graveyard shift at the Kennedy Center can be lonely. Aside from a few security guards wandering the grand foyers, there is little sign of life.

Inside the concert hall, the quiet is quieter still. There are no clocks, no windows, no shafts of light to help you gauge the passage of time. It’s in this void that Marchand and Fortin work away.

Their profession is an unusual one. They are organ voicers — please, don’t call them tuners — and probably not more than 100 people in the world do what they do. “You don’t dream of doing this when you’re young,” says Marchand, laughing. “People don’t know that this type of job exists.”

It is exacting work, demanding a precise ear and deep knowledge of every aspect of the organ. Marchand and Fortin have degrees in music and classical music theory and decades of experience. As organ voicers, they are responsible not simply for tuning the organ, but for developing the tone and musical color of the pipes, helping them breathe, giving them life, searching always for optimal output.

Marchand and Fortin’s employer, Casavant Freres, has been building organs since 1879, making it one of the world’s oldest organmakers. The organ voicers’ work begins in the company’s Quebec factory, where the pipes are cast and adjusted.

Each pipe’s sound is developed, made brighter or mellower, grander or more subtle. And each pipe is worked on not just for how it will sound individually but for how it will work in concert with the other pipes — from the five-eighths--inch pipe that can reach the highest note made by a piccolo to the 32-foot-tall wooden pipe that provides a note so low it seems to rise from deep within the Earth.

Marchand, 59, has worked for Casavant Freres for 24 years. Fortin, 51, has been there for 12. The work they do is specialized and has been been passed down generation to generation, voicer to voicer within the company, and going back centuries before that. The timelessness of what they do can cast a spell.

“The kind of job we’re doing today, we could have been doing 400 years ago,” says Marchand.

“Like getting into a new Mercedes’

The truck carrying this new organ and its thousands of pipes arrived at the Kennedy Center in August, and a crew immediately began assembling the intricate, 20-ton, multi-story arrangement.

Marchand and Fortin arrived a month later. They have worked on organs all over the world and are used to traveling and spending hours making the organs sound just right for the location. But this trip has been particularly long and grueling. The race is on to have the organ perfectly ready for its debut. Their visit to Washington has been spent almost entirely in the Kennedy Center and at the Fairfax hotel where they crash for the day after their night shift ends.

They have to take turns, alternating between playing the notes at the console and working on the pipes with their bag of specialized tools: knives, hammers and rods employed with great care to ever so slightly close or open the mouth of the pipe or alter the angle by which the air flows through it. They can’t listen to high pitch tones all night, so they alternate that as well. Otherwise their ears would be damaged.

To spend every night listening painstakingly to note after note might seem like draining, even tedious work. But the search for the sublime is its own reward.

“We are listening to that same sound over and over again trying to find just that one little change that will make it perfect,” Fortin says.

Marchand describes the process of voicing as a conversation. “The pipes are our boss,” he says. “They tell you what they want to do. We help them to do that.”

Actually, Fortin and Marchand’s boss is Jacquelin Rochette, artistic director of Casavant Freres, who has made a few trips to Washington to check on the progress. He is soft-spoken but passionate about the work the pair are doing.

“It is moving to be a part of this process,” says Rochette. “The voicing is the artistic part of this work. It gives the instrument its artistic soul.”

After an opportunity to rehearse with the new organ in late October, Neil, the NSO’s organist since 1985, was effusive with his praise.

“It’s like getting into a new Mercedes-Benz and putting the pedal down,” he said. “It is total pleasure. The sound just envelops you. Of course, this is subjective, but this may be truly the finest symphonic hall organ in the United States in terms of what it can do and how it will sound with an orchestra and with a chorus and even by itself.”

Marchand and Fortin watched the rehearsal offstage. Their pleasure, too, was hard to contain.

“It’s joy when it’s done,” says Marchand, with a wry smile. “Like a chef when you have to cook for 90 hours just for a 30-minute meal. Not all of the 90 hours are fun. But when we finally heard the organ play with the orchestra it was really great for us.”

“It confirms what we were thinking,” Fortin says. “That’s what we were working all night for.”

Kennedy Center Concert Hall Organ Debut: Free Concert 6 p.m., Nov. 27

READ MORE: Kennedy Center’s new organ arrives to much fanfare