Change in A-weighted sound pressure level or Noise level of radiated sound due to sound sources moving at low Mach number at the same speed along a straight track is discussed in this paper. When a sound source move, frequency and amplitude modulation is observed in the radiated sound field. Without their modulation, the noise level at a receiving point is determined only by distance and A-weighted sound power level of each sources. Solution of modulated frequency and amplitude of radiated sound can be obtained by using the Duhamels efficient calculation. The modulated frequency and amplitude increase for approaching sources and decrease for receding sources. The difference of maximum noise level,?and the equivalent sound level during the sources passing-by, with or without considering the modulation, increases monotonically with respect to source velocity, and independent of distance from the track. This difference increases as dominant frequency band of the sources decreases due to A-weighting below 1 kHz.