"For all the Huey’s advantages, it was perhaps its most famous characteristic
that proved its weakest link. The large, twin-bladed rotor created a loud, and
very distinctive, ‘whomp whomp’ sound in flight. This was due to the tip of the
blades breaking the sound barrier and creating a mini sonic-boom. This in
turn meant that the Viet Cong (VC) and North Vietnamese Army (NVA) could
usually hear Hueys approaching from some distance away."
Source: http://conflictarchaeology1.files.wordpress.com/2011/12/the-huey-helicopter-and-the-vietnam-war.pdf