There were no radio bulletins, no headlines and no breathless broadcasters reporting about it on the Six O'Clock News. But it's a significant story nonetheless, because it costs us as a community.

Metrobus lost two regular customers last week. Both have gone back to private autos--forever, they say.

The reason for the first defection: gum chewing on the bus.

The reason for the second: pot smoking.

We begin with H. Linda Humphrey, a yeoman in the Navy who lives in Alexandria and is assigned to the Navy Annex in Arlington.

Linda moved to the Washington area just five months ago, so she is precisely the sort of "new blood" that planners want to entice onto public transportation. And indeed, she immediately became a Metrobus regular. But then . . .

"Daily, I am the victim of SNAP! CRACKLE! POP! resounding in my ears," Linda writes. "I've wondered why people cannot chew their gum quietly and with their mouths closed.

"Plus, alas, the offenders are women. I'd like to tell them how unfeminine and ridiculous they look with their mouths open, cracking out their annoying concertos. . . . I often throw a sharp glance their way, but to no avail. They are obviously not aware of it, or just don't care.

"I know my only recourse is to stop riding Metro and end my misery, so: Goodbye Metro, Hello Solitary Driving.

"Thank you. I feel better already."

Sorry, Linda, but I feel worse. Those lip-smacking, cud-munching, self-indulgent creeps who forced you off the bus ought to consider this: The next time there's a fare increase, Metro will probably blame it in part on declining ridership.

And just why do you think ridership is declining, gummies? Because of you. Every Linda Humphrey you chase off the buses ends up costing you, the gum chewer, more money. So if you don't think you have a social reason to shut your overactive mouth, you ought to remember that you have a financial reason.

As for pot, Robert Andrews of Northeast calls to say that's why he has had it with the buses he used to ride from the Rhode Island Avenue subway station to his home.

"I get off work about the same time the schools get out," Andrews says. "I'd get on the bus and every day, in the back, there'd be a half-dozen kids with that look on their faces. But you could tell by the smell, anyway--it was marijuana. And nobody did anything, not even the driver."

Andrews has now arranged for his wife to pick him up at the subway station, even though it inconveniences her considerably. Meanwhile, Andrews wonders why regular pot-smoking can't be stopped by Metro officials. After all, unlike gum chewing, smoking anything on a bus is illegal.

"A bus operator has no powers of arrest," said Metro spokesman Cody Pfanstiehl, when I asked him about it. "If the smoker in the back is 6 feet 3 inches tall and the operator is 5 feet 7 inches tall, it is an uneven argument.

"If the operator stops the vehicle 'until that smoker leaves,' a bad scene can ensue and innocent passengers will be delayed and angry. This has happened.

"The most effective answer, and it sometimes happens, is for a 6-foot-6-inch passenger in the rear to suggest to the smoker that this should not be done on a public bus . . . . "

Incidentally, Pfanstiehl notes that Metro police do ride buses regularly, and do make arrests for smoking, be it marijuana or tobacco. "They made 2,609 arrests for smoking in 1981 on buses," he said. That's nearly eight a day--perhaps as much as we can expect from a chronically understaffed department.

Still, it is outrageous that Robert Andrews has been chased back into the family Oldsmobile--and that Mrs. Andrews has to hire a baby sitter for the 45 minutes it takes to make the round trip to the subway station.

Despite the chance of a "bad scene," I think the time has come for drivers to get tough and refuse to budge the bus if they smell that spicy smoke. Pot-smoking kids can get high anywhere, if they insist. Why does Robert Andrews have to suffer in the process?