Heating, ventilating, and air conditioning systems (HVAC) are today an important part of many people's life. They provide a sufficient amount of airflow with the correct temperature, quality, and humidity. The negative side is the noise it produces. Modern HVAC systems are driven by low-speed isolated centrifugal fans which produce the dominant tonal noises. There are no obstacles that the blades pass in this type of fan. However, the dominant tonal noise produced by the fan is at the blade passing frequency (BPF). This type of fan has a gap between the rotating shroud and the stationary inlet duct. Previous studies have shown that the flow through the gap causes turbulence that develops along the shroud wall and interacts with the blades at their leading edge. The interaction renders uneven surface pressure distributions among the blades as well as significant peak differences. The location of the tonal noise sources for BPF agrees with the locations of these interactions and surface pressure peaks. This study aims to investigate how the tonal frequencies are affected for different gap configurations. Two new designs are compared with a reference fan that was used in previous studies. The approach is to use the hybrid computational aeroacoustics (CAA) method, that couples the improved delayed detached eddy simulation (IDDES) method with the Ffowcs Williams and Hawkings (FW-H) equations. Our simulations show that the gap design effects the magnitude of the tonal frequency, BPF. By increasing the size of the gap, the amplitude can be decreased. The acoustic sources for the BPF agree with the location of the uneven surface pressure distribution on the blade leading edge. The time history of the surface pressure is therefore compared and it is shown that the gap configuration effects the amplitude of the pressure fluctuations. Also, the fan performance is affected by the gap configuration. The static pressure rise made by the fan and the pressure upstream of the fan in the inlet duct is influenced by the gap design.