What factor does not contribute to the noise level of an air-to-air heat pump?

In the context of air-to-air heat pumps, various factors contribute to the noise levels produced during operation. The compressor operation is known to generate significant sound, as it is one of the primary moving components within the system. Airflow also plays a critical role; as air moves through the heat pump's components, it can create noise that adds to the overall sound level. Mechanical vibration is another contributor because vibrations from moving parts can resonate through the structure of the building, leading to additional noise.

Thermostatic calibration, on the other hand, pertains to the settings and adjustments made to ensure the system operates at optimal temperatures. While improper calibration might indirectly lead to issues that could affect noise levels (like inefficient operation), it does not actively contribute to the noise itself during the heat pump’s operation. Therefore, it is the factor among the options that does not contribute directly to the noise level of an air-to-air heat pump.