Computer cooling fan noise is studied theoretically, focusing on the radiation from the interaction between rotor blades and motor struts. The source is decomposed into axial thrust, circumferential drag, and radial force. There is no sound-power coupling among the three components. The index of spatial spinning pressure mode plays the key role in noise radiation. The leading modes are the zeroth, or coincident, mode for thrust and the first mode for the drag and radial force. The effect of source noncompactness is quantified and found to be substantial only for higher-order radiation modes. The sound powers of the leading modes follow a sixth-power law, while the next high-order modes follow an eighth-power law. Quantitative analysis shows that the drag force can be equally noisy as the coincident thrust force. Based on an empirical aerodynamic model of rotor-strut interaction, it is found that the total sound power is more sensitive to the number of struts than rotor blades. Numerical examples are given to demonstrate how the struts can be optimized for typical cooling fan conditions.