If you were to stop the tail rotor blades in forward flight, and presuming the vertical fin could maintain sufficient yaw control, would the noise footprint of the entire helicopter, as heard from the ground, be significantly reduced?
The short answer is Yes.
During forward flight Helicopter noise is created by a number of processes.
- Thickness noise
- Low-frequency harmonic force noise
- High-speed Impulsive (HSI) noise
- Blade-Vortex Interaction (BVI) noise
- Broadband noise
Thickness noise is proportional to the maximum flux of fluid displaced by the blade section, so the main rotor (thicker) is more powerful than the tail rotor (thinner). Likewise, the low-frequency harmonic force noise is proportional to the low-frequency air loads on the rotor that produce lift. (The main rotor is also bigger in this case).
This being said, the main rotor generally produces energy with frequencies from ~10 - 100 Hz, which humans are generally poor at hearing. The tail rotor produces energy from ~60 - 400 Hz, which we are considerably better at hearing.
All of this is to say that stopping the tail rotor, though less of a decrease in energy than stopping the main rotor, is actually more likely to make the event quieter. In fact, many noise mitigations are based on tail-rotor noise, such as the NOTAR or a shrouded tail rotor both minimize such noise. (And also HSI and BVI noise.)
[Much of the information shared here is based off of Harvey Hubbard's "Aeroacoustics of Flight Vehicles, Theory and Practice: Volume 1 Noise Sources"]