There has been many reports of silent and unmarked helicopters throughout the United States. They have also been reports of apparent sightings of these "silent helicopters" in conjunction with UFO sightings such as the Cash Landrum encounter (http://en.wikipedia.org/wiki/Cash-Landrum_incident).
These helicopters use a McDonnell Douglas NOTAR (no tail rotor) technology (http://en.wikipedia.org/wiki/NOTAR) and they are referred to as MH-X Advanced Special Operations Helicopter as used in the assassination of Bin Laden http://www.globalsecurity.org/milita...craft/mh-x.htm. These technologicals makes these helicopters virually silent.
There has been strong evidence that these advanced helicopters have been used domestically in the United States in what is call black operations (http://en.wikipedia.org/wiki/Black_operation), some directed at private citizens. They have on board sophisticated technology including infrared, and advanced listening surveillance equipment. It has also possible some form of ultra sonic weapon installed.
There has been numerous photographs of these are other types of helicopters being used for domestic intelligence gathering, including on those who have had closed encounters, and have reported UFO sighting. With the domestic unrest in the United States they usage will be increased dramatically.