Disturbing new signs of terrorism by Iran's Revolutionary Guards lie at the heart of U.S. concerns that Silkworm missiles in the Persian Gulf may prove lethal in their hands.

The Guards are swaggering street-fighters, haters who fire from the hip. They answer only to the Ayatollah Khomeini, and he regards them as his special heroes.

Even today, with the Iranian revolution devouring its children, the regular armed forces have managed to maintain strict discipline, ruling out attacks on the United States or other Western powers. Not so the Guards. Their speedboats, built in Sweden, make hit-and-run attacks on Gulf shipping. It would be natural for the Silkworm launching site now under preparation for the Guards at the Strait of Hormuz, the entrance to the Gulf, to be used with similar indiscretion.

That risk, which would simply enhance these other recent acts of terror by the Guards from Tehran to Paris, has been accepted by President Reagan's national security high command. However, elsewhere there are doubts. These doubts have been growing since private White House polling after Reagan's Monday night TV address showed, in the words of an administration official, that he ''did not score well'' on his Persian Gulf policy.

White House Chief of Staff Howard Baker, who catches cold when senators sneeze on Capitol Hill, quietly advised the president not to rush into putting U.S. flags on 11 Kuwaiti tankers until the fractious congressional view crusts over favorably and public opinion turns. ''Wait,'' Baker advised. A major reason for that advice: the hair-trigger Guards will make prophets of congressional critics predicting that American blood may be shed in the Gulf.

But delay would run worse risks, intrinsic because of U.S. responsibility; political because it would cast new doubt on the president's ability to call the hard shots in the twilight of his presidency. National security aides are telling Reagan the United States cannot base policy on threats from the Revolutionary Guards.

The rising tide of their terrorism peaked in Tehran with threats to cut off the hand of British diplomat Edward Chaplin three weeks ago. Chaplin's sin was to drive his family home through the streets of the capital. He was lucky to keep his hand, but he was roughed up before Britain's angry demand for his release got him out.

There was a nightmarish quality about this assault, from its unpredictability to its stupidity. That is what makes the Reagan administration so aware of the Silkworms soon to be installed at the entrance to the Gulf. The assault was carried out by an unstructured Guard unit, which claimed it had evidence Chaplin was recruiting ''agents'' inside Iran.

The Guards have targeted many other foreigners, including a West German diplomat, an Australian, an Italian and, despite the Iran-Syrian alliance, a Syrian diplomat. That demonstrates the danger implicit in their gaining control of a Silkworm launching site overlooking the Strait of Hormuz, the choke-point entrance to the Persian Gulf.

The need for American power to provide safeguards for oil tankers owned by friendly Arab states such as Kuwait, carrying oil to friendly allies such as Japan, seems self-evident. Given the prospect of uncontrolled maritime banditry and unpoliced lethal power directed at peaceful oil traffic from line-of-sight launch sites, it would seem a dereliction for the United States to stand back.

What Howard Baker is now hearing inside the administration is this: the president's decision in this matter supersedes negative public opinion, and should not be dictated by congressional critics. But the issue is not yet finally decided.