But now the phantom voice has taken center stage in the latest flurry of claims and counterclaims between Iran and the United States following a tense high seas confrontation - raising new questions about whether Washington could have gotten a key element of the story wrong.
The radio transmission - a staccato burst suggesting U.S. Navy ships were targeted for explosion - was a central part of an audio-video presentation that U.S. officials claimed showed Iranian speedboats swarming three Navy warships on Jan. 6 in the Strait of Hormuz, the narrow waterway that is the only entry and exit to the Gulf.
Iran has called the U.S. recordings fabrications. Tehran's rebuttal received fresh attention after a newspaper that focuses on Navy issues said veteran U.S. sailors believe the threats could have been a well-known Gulf gadfly who has been pestering Gulf ships since at least the 1980s. There is also the possibility that more than one broadcaster has been contacting ships over the years.
Such renegade broadcasters are called by the ethnically offensive moniker, "the Filipino Monkey" - a name used by mariners around the globe for someone who uses a radio for unnecessary or inappropriate transmissions.
The questions raised in the Navy Times on Sunday also come at a sensitive time for Washington as President Bush visits Gulf nations and presses for greater unity against Iran's attempt to expand its influence in the region.
Seagoing exchanges between U.S. and Iranian vessels are not uncommon in the crowded Gulf shipping lanes, especially near the Strait of Hormuz where Iran's coastline is within miles of international waters.
Last week, U.S. Defense Secretary Robert Gates said there had been two or three similar incidents over the past year, but "maybe not quite as dramatic" as the Jan. 6 confrontation. None had been publicized until the eve of Bush's trip to the region, even though in one, in December, a U.S. ship actually fired warning shots toward an Iranian boat.
In the Jan. 6 incident, the Pentagon said U.S. Navy commanders also were considering firing warning shots, before the retreat of the five Iranian speedboats, which the Pentagon said were operated by the elite Revolutionary Guards.
Washington released a tape of a radio transmission of a male voice speaking in heavily accented English on an open frequency: "I am coming to you. ... You will explode after ... minutes." The White House called it a dangerous act, adding that the Iranian sailors also dropped boxes into the sea near the U.S. vessels.
"Inbound small craft: You are approaching a coalition warship operating in international waters. Your identity is not known; your intentions are unclear," the unidentified Navy crew member says on the tape. He then cautions the Iranians that they would be "subject to defensive measures" if they did not pull back.
The airwaves on the sea are full of rogue broadcasts. Many are dismissed as harmless snippets or remarks, but the one heard during the encounter with the Iranian craft was taken seriously.
Still, the Navy has said it cannot pinpoint the source. Over the decades, neither the location or the nationality of the Gulf radio prankster - or pranksters - has been determined.
Other evidence also has cast doubt on whether the threat came from the Iranian boats, including a lack of background noise - such as boat engines or wind - on the audiotape. Other analysts have noted the voice does not sound like an Iranian accent.
Cmdr. Lydia Robertson, spokeswoman for the Fifth Fleet in Bahrain, said Monday that the Navy - while trying to determine the transmission's source - still believed the overall incident was highly provocative.
"The Iranian boats were coming close to the ships, making aggressive maneuvers and objects were being dropped into the water," she told The Associated Press.
Cmdr. Jeffery James of the destroyer USS Hopper, one of the U.S. warships involved, said Sunday that it was the confluence of factors that caused most alarm.
"Whether it (the radio threat) was coincidental or not, it occurred at exactly the same time that these boats were around us, and they were placing objects in the water - so the threat appeared to be building," James said.
Joseph Cirincione, of the liberal Center for American Progress in Washington, said the Bush administration could stand accused of having "needlessly hyped a threat for political purposes" if it's determined the radio voice was not from Iranian forces.
Another danger is weakening U.S. credibility among European allies the United States needs to maintain a tough line against Iran on standoffs such as Iran's nuclear ambitions and its suspected aid to Shiite militias in Iraq. France, Germany and Britain are likely to stay allied with Bush on Iran, but only as long as they believe the United States has compelling evidence of Iranian wrongdoing.
U.S. sailors say they have heard the prankster - who is possibly more than one person - transmitting "insults and jabbering vile epithets" on unencrypted frequencies during Navy exercises in the Gulf for years, said the Navy Times, a privately owned newspaper.
"Navy women - a helicopter pilot hailing a tanker, for example - who are overheard on the radio are said to suffer particularly degrading treatment," the newspaper said Sunday.
Rick Hoffman, a retired Navy captain, said a renegade talker repeatedly harassed ships in the Gulf in the late 1980s when U.S. warships protected Kuwaiti oil tankers during the Iran-Iraq war.
"For 25 years there's been this mythical guy out there who, hour after hour, shouts obscenities and threats," Hoffman told Navy Times. "He could be tied up pier-side somewhere, or he could be on the bridge of a merchant ship."
The Iranians frequently send frigates and patrol craft or reconnaissance planes, including U.S.-made P-3 Orions, to watch U.S. ships in the Gulf. The Navy often responds by scrambling jet fighters to intercept and shadow Iranian planes.
Said Cirincione: "We have to take a step back and make sure we don't hyperinflate these threats, to prevent a shooting war that nobody really wants."
Mickey Gurdus, an Israeli who has been monitoring radio and TV broadcasts for Israel Radio for four decades, believes the Gulf broadcast was not a hoax and "could have been psychological warfare."
"I can't imagine that there is anyone in the proximity of the Strait of Hormuz that would carry out a hoax like that," said Gurdus, who did not monitor the original radio broadcast Jan. 6. "I think this was something real."
Gurdus said he believes the broadcast heard by the U.S. vessels were UHF or VHF transmissions that would originate within a 60-mile radius of the strait if made from land and 180 miles from an airborne transmitter.
Michael O'Hanlon, a senior fellow at the Brookings Institution in Washington, said he was surprised that the source of the transmission remains unclear, because of the many decades of U.S. naval presence in the Gulf.
"You would have thought we would have been able to nail this down," he said.