You are walking down the street, queuing in a shop, or sitting in a restaurant, when it begins.
The growling comes first, an animal-like noise that makes you wonder for a moment if there has there been an escape from Beijing Zoo.
This is followed by an awesome clearing of the throat, a sound akin to a plane taking off.
Then the climax. Phlegm is broken and with great gusto it, a spit, is launched. It usually lands close enough to get your stomach wrenching. It happens often and everywhere in China.
There we were yesterday buying a Christmas tree in the famous Liangma Flower and Tree Market, when the spit of a Chinese customer went whistling past our ears down the stairs.
Recently we were out for a stroll when my 12-year-old son started to show off his newly acquired spitting skills. After a serious admonishment he replied, "But everyone does it." Your first instinct when you see someone about to spit is to shout your disgust, and demand that they stop. Then you remember you are a foreigner in a country where spitting in public is no big deal.
A recent survey showed that 300 million Chinese, or one quarter of the population, spit in public. More than 70 per cent of those polled thought that a little projectile clearing of the throat was no big deal. Nearly 20 per cent said spitting was a natural, healthy activity that they would be uncomfortable giving up.
Changing such an ingrained habit will be tough. But the mayor of Beijing, Liu Qi, feels the time has come to get the Chinese public to mind its manners.
The mayor is not just concerned about spitting. He has launched a campaign to wipe out a dozen bad habits that might embarrass China during the Olympic Games to be held in Beijing in 2008. Spitting is top of his hit-list, ahead of cursing, jumping queues, pushing on trains, blocking passengers getting off the subway, never shutting up, refusing to apologise and never smiling.
"We need to build a new image of Beijing," he said recently. "Although these habits are just foibles, they reflect a major flaw of our spiritual civilisation."
Mayor Liu is only the latest in a long line of Chinese officials who have tried to clear the streets of Beijing of phlegm. Even before the Communists took power in 1949 with plans to reshape human nature, the nationalist government sent soldiers through the capital to try to stop residents from spitting wherever they pleased.
In the early 1980s, the former Chinese leader, Deng Xiaoping, launched a massive campaign against "this unhealthy practice". He sent an army of 200,000 health inspectors to levy fines on spitters in Beijing alone. It probably didn't help that Deng, the architect of China's market reforms, was himself known to keep a spittoon near his chair in the Great Hall of the People. It is said he often punctuated his sentences with a well-aimed shot of spittle, to the chagrin of the foreign dignitaries who came to see him.
One thousand anti-spitting centres were set up around the city, with microscopes showing the dangerous bacteria in sputum and banners bearing slogans such as, "Keep fit. Don't spit."
In the early 1990s, as newly prosperous Chinese began travelling overseas, the government published a booklet on proper behaviour while abroad. "Do not spit in public. If you must do so, spit in your own palm," it said.
However these campaigns have produced only limited results. Wang Liu is one of hundreds of uniformed inspectors who patrols Beijing's streets looking for spitters and litterbugs. He says the situation was much worse 20 years ago. "There was spit all over the place," he said.
Wang catches several offenders a day. Their punishment? He can fine them up to 5 yuan - 50p. But he prefers to order them to bend down and wipe up the mess and gives a lecture about how spitting spreads disease, soils the environment and embarrasses the motherland.
According to Duan Feng, a trader in Silk Alley market, spitting has been part of the habits of Chinese people for centuries.
He pointed out that the bad air quality is one of the main reasons that people feel compelled to clear their throats of phlegm so often.
"You can't blame people for doing what they have to do. What's the fuss?" he said. However, Wu Su, a guard outside the American embassy, said spitting is a habit he would love to see eliminated. "It is disgusting and gives people a bad impression of the Chinese."
A Beijing sociologist and market researcher, Victor Yuan, who has studied Chinese attitudes toward spitting, said traditional Chinese medicine regards phlegm as a symptom of a natural imbalance in the body, so spitting is considered a healthy act.
Yuan said it was only after Westerners arrived in China during the 19th century and complained about routine public spitting that the Chinese began to see it as a problem.
So the moral of the story is when you visit Beijing watch out . . . you don't want to be hit by it.
miriamd@163bj.com