Reagan administration suspicions, fed by reliable Iranian sources, are once again running high that Israel or Israeli-linked private arms dealers are moving a vital component of the Hawk anti-aircraft missile to Iran to help it defeat Iraq, the arch-enemy of both countries.

Israel's surreptitious sale of arms, disclosed in the Iran-contra investigations, is a matter of record. Before those probes ended, exposing Israel as the most intense source of pressure on the administration to sell Hawks and other arms to Tehran, Israel promised to stop all arms shipments to America's No. 1 enemy.

But one top administration official, his suspicions fed by new reports he acknowledged are difficult to prove, told us privately the Israeli pledge is now ''suspect.'' That means Israel may be back in the business of arming the country the United States is striving to isolate from foreign arms and to target with United Nations sanctions if it rejects the Security Council's cease-fire demand.

An Iranian source with close ties to the Tehran regime says privately the Iranian army is running out of a vital part of the Hawk anti-aircraft missile, called Transport Radio Communication 145. The Iranian, interviewed in New York, says Iran has been buying replacements from what he called ''our reliable source, Israel.''

Without the replacement, the unimproved Hawk missiles -- similar to the first shipment of Hawks illegally shipped to Iran by Israel on Nov. 22, 1985 -- are useless. With Iraq stepping up its air attacks on oil-loading docks and other Iranian targets, Hawks may be more essential to Iran now than they were in 1985.

U.S. officials are wary when asked whether Israel is back in the guns-for-Iran business. That is partly because Israel is regarded as a close ally of the United States and partly because the Israeli lobby is so powerful on Capitol Hill. The lobby's top target today is President Reagan's new arms package for Saudi Arabia, which is helping the United States protect Persian Gulf shipping.

Suspicions at high administration levels are growing on the arms-for-Iran question. ''There are persistent reports about this, but the Israelis deny it, and we still cannot prove it,'' a State Department spokesman told us. A high Pentagon official, asking anonymity, told us: ''We suspect it.'' He said Secretary of Defense Caspar Weinberger ''has really leaned on Israel not to undercut U.S. policy.''

The Hawk missile is made by Raytheon and is not licensed for general export. No U.S. ally that has it can legally export it or any of its parts. No license has been granted to Tehran.

If Israel is helping Iran, the problems of proving complicity in arms sales are massive, possibly insurmountable. Illicit arms sales can be concealed behind dummy corporations, fraudulent documents, doctored export licenses, laundered money and the official lies and pledges of innocence that poured from the lips of U.S. and Israeli officials a year ago.

Highly esteemed Israeli Foreign Minister Shimon Peres solemnly informed leading Washington reporters at breakfast in the Willard Hotel on Sept. 16 last year that Israel was shipping no arms of any kind to Iran. That was the month of Israel's first big sale of TOW antitank missiles and only shortly before its first shipment of Hawks left Israel. When we asked an Israeli Embassy official this week if Israel is selling arms of any kind to Iran, he gave us a flat, unequivocal denial.

Hawk parts, similar to Israel's 1980 sales of tires for the F-4 that kept Iran's air force flying, are not the only mystery facing the administration about Iranian arms. New evidence reaching Washington strongly disputes the theory that Communist China has been the direct source of Iran's dangerous Silkworm missiles, which are capable of sinking U.S. naval vessels patrolling the Persian Gulf. Although these missiles are known to have been made in China, American specialists are now convinced they got to Iran by circuitous routes, raising the question of third-country culpability.

Administration officials believe two of Israel's strategic goals are Iran's victory over Iraq and the prevention of close U.S.-Arab relations. Helping Iran obtain battle-worthy Hawk missiles and possibly other arms is a key to the first. The defeat of President Reagan's new arms package for Saudi Arabia is a key to the second.

The White House is preparing to fight Israel on the Saudi arms package. That will pale next to the political battle against Israel by the administration if it can find hard evidence to prove what it suspects about Israeli complicity in arming Iran. .