This study discusses the development of a tone noise model dedicated to an open fan engine, also called Unducted Single Fan. Because of its particular architecture, this engine aims to reduce fuel consumption, but it might increase the radiated noise due to the fairing removal. This model intents to be soon implemented into acoustic source mapping techniques to better characterize the dominant sources in future wind tunnel experiments. It is based on Ffowcs-Williams and Hawkings analogy relying on the thickness and loading noise terms of Goldstein’s formulation, written in the frequency domain. A decomposition into angular modes through equivalent fixed phase-shifted sources is proposed. The predicted sound field takes in account different contributions, which are investigated here: the thickness noise, the steady and unsteady loading noise (from rotor and stator) in forward flight configurations, and including near-field effects. The unsteady loading harmonics are obtained from URANS solutions. The present predictions are validated by comparison with time-domain FWH experienced solver and CFD inputs from elsA ONERA code, at multiple operating conditions. Two specific cases are addressed: firstly with fully non-compact sources involving a complete discrete surface integration, secondly with the sources compacted in chord and distributed over radial strips.