Fifteen years ago, says Jerusalem's amiable Mayor Teddy Kollek, there was a popular joke here about two Americans on their first trip to Israel visiting a nightclub in Tel Aviv.

One of the Americans joined in the uproarious laughter of the Israeli audience watching the show.

"Do you speak Hebrew? His puzzled friend asked.

"No," said the laughter, "But I trust them."

Today, observed Kollek, who recently toured the United States to promote his autobiography, Americans are not nearly so trusting. They are better informed, he said, asking harder questions and expressing doubts about Israel's policies in pursuit of Middle East peace.

Even among American Jews, he said in a conversation the other day, the era of the blank check is over: "They want to know just where their money is going . . ."

As Israel marks its 30th Anniversary of statehood, that change in its relations with Americans and the U.S. government is an important theme, one of the reasons that many visitors here for the festivities have been struck by a somewhat somber edge to the occassion, an underone if introspection.

There is, of course, another big factor behind the subdued modd: disappointment that last fall's visit to Jerusalem by Egyptian President Anwar Sadat has so far produced no concrete steps toward Arab-Israeli accord, thus raising only the wearisome prospect of more tension. Taken altogether then, Israel this week is feeling its age.

"One day she awoke and she was over 30," noted the gifted catoonist Kuschen in an anniversary allegory for the Jerusalem Post. "She longed wistfully for those innocent days of her youth, when she was the brave little, democracy in the Middle East . . .

"But she was 30 now . . . and He (over a sketch of Jimmy Carter in an uncle Sam hat) suggested that she "reduce a bit. Once he loved her 'chutzpah' but now it was just instransigence." Once the politicians of the world had sent her bouquets. But now she pressed each solitary rose between the pages of a book . . .

"When her first mature flirtation (over a sketch of Sadat) didn't end at the altar after a two-week fling . . . She was crushed. She knew that she had behaved like a giggling 16-year-old . . . And now she didn't know if it was over nor . . . or if it had only been a game for him.

"But she could see things more clearly now . . . She'd never be a sexy young thing again. But she would be herself, a little older, a little wiser, a bit more cynical. Someone with a past . . . and a future. A good future."

THE POINT ABOUT Israeli sobriety should not be overstated, however. Wednesday night when Independence Day began, The downtown streets of Jerusalem filled with thousands of people. Many danced the traditional hora. Later, Israelis gathered at the homes of friends and sang songs, in some cases until dawn.

Throughout the hubbub of the evening, as has been the holiday custom of recent years, most youngsters were armed with toy plastic hammers that squeaked. They would bop passersby in a gesture of goodnatured familiarity that is, quite literally, touching.

The next night, in the Sultan's Pool, a large basin at the foot of the ancient walls of Jerusalem's old city. A spectacular outdoor concert was mounted, featuring among others, Mstislav Rostropovich, Isaac Stern and Leontyne Price with the superb Israeli Philarmonic conducted by Zubin Mehta.

The planned finale was a rousing rendition of Tchaikovsky's "1812 Overture" complete fireworks and a cannon blazing at the finale. Such are still the conditions of life in Jerusalem that for several days in advance newspapers assured residents not to be concerned by the explosions.

Then came a last rendition of "Hatikva," the hauntingly melodic Israeli national anthem, led by Price's glorious soprano. Moved by the moment and the beauties of the setting, many spectators wept. Israelis may indeed have been chastened a bit by their troubles but they are not less sentimental.

Nor, it should also be said, are all or even most Israelis willing to accept without rancor the pressure for flexibility in Middle East peace talks that is coming from Washington. Government loyalists - according to one as yet unpublished poll, the foreign politicies of Prime Minister Menachem Begin recently drew 70 percent support - say here are limits to what Americans should expect from Israel.

"There is one thing for sure," said one very senior official, "we are not going to give up our lives and the existence of Isreal to make it easier for you in the United States, for the convenience of American citizens. You can tell that to Mr. Carter and to Brzezinski," the president's national security adviser.

"One day," continued the official, who did not want to be named, "we will arrive at the red line and Israel will give a clear and definite answer that it shall go no further. The result could be war. We will do everything for peace. But we will fight another 10 wars if we have to.

"We are never going to come to a situation in Israel where our security will not be in our own hands."

THE FAMILY OF Emmanual Bychowski - he and his wife, Bella, two daughters, a son and seven grandchildren - live about half a mile from the Jordanian border on Kibbutz Maoz Haim. For them the contentious matter of security is no abstration. Until 1970 the Kibbutz was regularly shelled. Their cottage was hit twice. There still are bunkers everywhere. Emmanual's son-in-law, husband of his eldest daughter and a major in the tank corps, was killed in the Golan Heights on the first day of the 1973 Yom Kippur war.

Bychowski came to Palestine as a boy and was one of the founders of Maoz Haim in the 1930s.He worked with the cows until a few years ago. Now he translated and edits books from Russian. His son Moshe has taken his place with the cows.

The most appealing thing about the family, aside from a ruddy health and vigor, is their thoughtfulness. Despite the isolation of their location, they know and think about the most intricate details of the Middle East situation. They are not strong Begin supporters but believe that he is as qualified as any Israeli leader to end animosity with the Arabs.

They wonder about Jimmy Carter's commitment to Israel, but they understand that the United States must be the arbiter for the peace that will eventually come. Emmanual, in particular, argues a philosophical view. Recalling the excitement of Sadat's visit to Jerusalem, he said, "For 29 years we lived with no hopes for peace. Now at least for one year we have had some hope."

It is a hope that he and his family need to cling to firmly.