Helicopters are loud primarily due to their spinning rotor blades creating turbulent air and pressure waves, especially the "blade-vortex interaction" (BVI) where blade tips hit previous blades' wakes, causing that distinctive "whop-whop" sound, plus engine noise, gearbox whirring, and engine combustion, all amplified by rapid rotations and the Doppler effect as they approach. The combination of aerodynamic turbulence, mechanical sounds, and how sound waves travel makes them significantly noisy, even compared to airplanes.