In the unmistakable sounds of Lebanon's capital by night, cars in hot pursuit whined by and gunfire cracked up and down the narrow street outside the hotel.

In the lobby, where some foreigners were chatting, several recent arrivals and a pair of nervous old-timers bolted behind overstuffed sofas, fearful that stray bullets might crash through the plate-glass doors. As they cowered on the carpet, the veteran hotel manager, suave and cool in a blue suit, ambled by on his way to see what the fuss was about.

He stood in the street as the shooting faded into the distance, but no one ever found out what happened or why. By the time he came back inside, normal conversation had resumed--on the likelihood of an all-out Israeli invasion of Lebanon, for example, or which restaurant to have dinner in.

As such violence tightens its hold on life in this disintegrating nation, the reasons for bloodshed are becoming so obscure--and often meaningless--that Lebanese and their foreign friends lose the thread and, eventually, stop being interested.

Even the threat of an Israeli invasion, repeatedly brandished by leaders of the Jewish state in public statements, has lost its urgency here, devalued by frequency.

The government of President Elias Sarkis seemed to encourage its citizens' detachment the other day by officially informing the United Nations that it can do nothing about any Palestinian guerrilla attacks on Israel launched from Lebanese soil, a stark recognition of its impotence before the firepower of the Palestine Liberation Organization.

Against this background, Deputy Assistant Secretary of State Morris Draper flew into Beirut recently for talks with Sarkis and his government in what was billed as another U.S. effort to salvage the teetering 10-month-old cease-fire between Israel and PLO forces. On the eve of Draper's arrival, Premier Shafiq Wazzan pointed out that Lebanon is not a party to the truce. This hung a pointed question mark over the utility of the talks.

Michel Abu Jawdeh, editor of Beirut's respected An Nahar newspaper, underlined this anomaly in a column, noting that the fate of the cease-fire actually lies in the hands of Prime Minister Menachem Begin's government and Palestinian guerrilla commanders, with Lebanon reduced to the role of a bystander.

PLO leader Yasser Arafat, who was a party to the cease-fire, was in the meantime on a visit to India. In any case, Draper was barred from conferring with PLO officials directly because of a seven-year-old U.S. pledge to Israel not to deal with the guerrilla leadership until the PLO recognizes the Jewish state.

The guerrillas' number two commander, Salah Khalaf, has been tied down by a outbreak of separate violence in the southern port of Sidon, where PLO irregulars and leftist Lebanese sympathizers operate a nearly independent community.

As is increasingly the case here, the cause of the fighting was unclear. Its results, however, were all too clear: four persons killed, 11 wounded and widespread damage to shops and homes by rocket-propelled grenades and light artillery.

The clashes matched PLO guerrillas against local Moslem militias united in the Popular Nasserite Organization. Palestinian guerrillas already are in frequent clashes with Shiite Moslem irregulars elsewhere in the southern Lebanese hills, so the PLO can ill afford to break its alliance with the predominantly Sunni Moslem leftists who help it manage Sidon.

Walid Jumblatt, over-all leader of the Palestinians' leftist Moslem allies, underlined this danger in a statement pointing at the Sidon fighting as a "crisis of confidence." "We must tell those responsible in the Palestinian resistance and others that we Lebanese have proved to be your protective shield," Jumblatt said. "But you must respect the ambitions and resources of this homeland."

Khalaf, who is also called Abu Iyad, pledged to organize compensation for shopkeepers whose establishments were damaged and set up a special tribunal to hand out immediate justice to the guerrillas and leftist militiamen whose street quarrel apparently led to the fighting.

But as these measures restored a comparative calm to Sidon, militiamen from the Nasserite Murabitoun forces in Beirut fought a running gunbattle--more than a week ago now--in a major West Beirut artery with gunmen from the Lebanese Communist Party. Again, the cause of their shootout was obscure, but leaders of the nominally allied groups met during the evening to patch up the dispute.

Several Beirut newspapers published photos of Murabitoun leader Ibrahim Quleilat and Communist leader George Hawi. But the gunfight was hardly mentioned. None seemed to wonder why it happened.

Clashes that have taken several dozen lives in the last two months in Tripoli also match two Lebanese groups. That fighting, however, seems to flow directly from attempts by the 24,000-man Syrian peace-keeping force here to impose increased control on Palestinian-backed local militias in Tripoli.

Tripoli gunmen, by long tradition fiercely independent of any authority, particularly resent a new Syrian-sponsored political party and militia that Tripoli residents charge have carried out interrogations.

Fighting between the street gangs and the militia at times has been intensive. On the day Israeli warplanes last bombed PLO positions, May 9, the Tripoli clashes left 17 killed and 80 wounded, compared with 12 killed and 40 wounded in the Israeli air raids.