IT IS 9 o'clock on a Monday night, and the musicians of the Fairfax Symphony Orchestra are taking a short break. The band room at Lanier Intermediate School has been oppressively hot, the rehearsal intense. But as they burst noisily through the doors and into the hallway, they are confident, relaxed. Their immediate surroundings -- water fountains two feet high, crayon drawings on the walls -- begin to take effect, and they joke and tease each other like kids.

The consensus tonight seems to be that the members of the "low brass section" are the class clowns. "We're the farthest away from the conductor, and we've got very little to play," says trombonist Paul Schultz. "What else are we supposed to do?"

Many of the orchestra members are professional musicians or music teachers. Others are lawyers, account executives, government and military personnel, computer analysts. Musical director and conductor William Hudson has brought them together in search of excellence, and they have gladly accepted the challenge.

One moment in Hudson's presence leaves a distinct impression. With his sharp features, intense gaze and imposing manner, he seems a man of serious purpose -- and in Hudson's case, such first impressions are not deceptive. In his 13 years as director, he has molded a "good community orchestra" into one of formidable accomplishment and reputation.

"When I arrived," Hudson says, "I began a policy of re-auditioning every year to eliminate weaker players. I have continued to do so, although most of them aren't weak anymore." Founded in 1957, the Fairfax Symphony now attracts some of the best talent in the Washington area, and turnover within the 110-member orchestra is minimal. Although this year's auditions attracted 50 hopefuls for 17 openings, none of the orchestra's previous members were replaced.

Tim Wade, a librarian with the Army, is also a first violinist with the orchestra. He is quick to point out that his involvement is much more than a hobby. "Many of us don't make the majority of our income from playing," he says. "But we do train professionally. We are in a unique situation -- if we were taken and moved to an area with less competition, we would double in stature. As it is, we find ourselves up against organizations like National and Wolf Trap, and audiences are less willing to give us a chance."

These musicians know they are talented. A primary goal these days is to let everyone else know it. To help usher in its 1984-85 season -- which begins on Oct. 28 at the Kennedy Center Concert Hall with renowned cellist Janos Starker as guest artist -- the orchestra is hosting a promotional and fund-raising 10K run this Sunday at 9 a.m., starting in McLean at 8200 Greensboro Dr. Participants are being urged to leave their Walkmans at home -- because live music will be provided. A German band, a horn trio, a solo harp and a suzuki string quartet will be stationed at various intervals along the course to spur the runners on.

Julie Anderson is a first violinist with the orchestra as well as a violin teacher at home. "We're here because we want to be," she says. "You wouldn't believe the amount of time we give up, but it's worth it!"