The Mystery Helicopter Reader Luxembourg sends several links to articles that say the helicopter that was shot down in the OBL raid as likely a long-rumored stealth UH-60. There was enough of the tail assembly left to make the identification. The wreckage, says Luxembourg is likely on its way to China. The now cancelled RAH-66 Comanche had stealthy features. As for making helicopters quiet, this is old stuff. We recall someone telling us after the end of Indochina II that the US has OH-6s that made about as much noise as wind blowing leaves. Here's a history article http://www.airspacemag.com/military-aviation/the_quiet_one.html
He carried out various oil and gas and electric transmission line studies notably the ADB sponsored CASA 1000 in 2008 and World Bank sposnored re-routing of CASA 1000 at Salang Pass as well as the UAP Line initial study.