We demonstrate a broadband and flat millimeter-wave (MMW) noise source based on the heterodyne of two Fabry-Perot lasers subject to optical feedback. Different mode intervals between two lasers are designed to generate beat terms at specific frequencies. As a proof-of-concept demonstration, a MMW noise signal with a 3-dB bandwidth of 50 GHz (limited by the measurement bandwidth) and flatness of less than 2.9 dB is experimentally achieved. The physical origination of the broadband flat MMW noise generation is analyzed, and the properties of the MMW signal are characterized. The proposed method has the potential to generate a broadband flat noise signal in the MMW or even the terahertz region.