[finnman69]

they claim they are quieter than a Cobra or Chinook. Obviously its not an Apache.

http://www.globalsecurity.org/military/systems/aircraft/v-22-survive.htm

Compared to both helicopters and conventional turboprops, the Osprey has a lower acoustic signature due to the tiltrotor's reduced rotor rotational speed. It also uses very low thrust for cruise propulsion. The V-22 flying in aircraft mode produces a distinctive sound, described by observers as a "throaty and muted hum - more like a vehicle than a helicopter." The observers noted that, in combined operations, the steady buzz of the MV-22 was frequently masked until the last minute by the "whop-whop noise" of AH-1 Cobras and UH-1 Hueys that were supporting nearby. Overall, as compared to the CH-46, the MV-22 is less noisy while in the aircraft mode, and provides comparable acoustic acquisition cues while operating in the helicopter mode.

[azhenfud]

The observers noted that, in combined operations, the steady buzz of the MV-22 was frequently masked until the last minute by the "whop-whop noise" of AH-1 Cobras and UH-1 Hueys that were supporting nearby

Ah! I see! When I was observating them, I had no "whop-whop noise" of AH-1 Cobras and UH-1 Hueys confusing my ears.