By Andrew North
BBC News, northern Kuwait
There was no mistaking the rhythmic thudding noise.
Helicopters may be heard not seen
A large helicopter was heading in my direction from somewhere above the Kuwaiti desert.
But no helicopter appeared.
It never would have done, because the sound was coming from a powerful speaker mounted on a military vehicle belonging to one of the US Army's psy-ops, or psychological operations units.
They have been attached to a 7,000-strong US marine unit called Task Force Tarawa, based in northern Kuwait.
If I had not known I would have been convinced it was a real helicopter.
This is just one of a myriad of different sounds they have on board that they can play to confuse or deceive potential adversaries about US intentions.
And these will be put to use if this unit is ordered north into Iraq along with the Marines.
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"When we see an Iraqi position", explains Sergeant Dan Voss, "we can make it appear there is an extremely large US force nearby, playing the sounds of tanks for instance or even of a group of US marines charging."
He will not say what other techniques may be employed in any conflict with Iraq except to say they have more than 100 from which to choose.
Tried and tested
Some of America's psychological operations on Iraq have been going on for some time.
Regular air drops of thousands of leaflets are aimed at Iraqi troops urging them not to obey orders and surrender.
Others are pitched at civilians and give details of pro-US radio stations or where the US believes they will be safest in any conflict.
Psychological operations really proved themselves in the 1991 Gulf war, according to US officials here.
The island was taken without a shot fired
Thousands of Iraqis were persuaded to surrender rather than fight US forces, through a combination of leafleting and mobile sound systems.
Sgt Voss gives the example of Kuwait's Failakah Island.
Psy-ops units flew around the island telling Iraqi forces if they were not in a specific location at a specific time, with their weapons left at their base, they would be attacked by US marines.
"Next morning, to our surprise, 1,400 people including an Iraqi general did exactly as we asked them to do.
"And the island was taken without a shot fired."
We sounded up our loud speakers and played some music for the marines to give them a morale boost
Military officials here believe they could pull off more dramatic successes like this again should an attack on Iraq go ahead, because even more of America's 10,000-strong psy-ops force is being deployed.
The challenge then, as in 1991, will be in dealing with the thousands of Iraqi prisoner of war.
Psy-ops units also played a key role in the US takeover of Panama in 1989 when American forces ousted Manuel Noriega.
When he barricaded himself in a building, psy-ops units bombarded it with loud music until he gave himself up.
This Kuwait-based US psy-ops unit also has plenty of music on board if they want to annoy anybody.
But in this isolated desert camp they have found being able to play loud music also has more conventional uses.
"The other day we sounded up our loud speakers and played some music for the marines to give them a morale boost," says Staff Sergeant Gerald.
So what did they play?
"Highway to Hell and Back In Black by AC/DC.
"We do take requests though."