Reactor noise in accelerator-driven systems (ADSs) is different from that in critical or radioactive source-driven subcritical systems because of the periodically pulsed source and its non-Poisson character. In two earlier papers, we developed a theory of ADS reactor noise, incorporating these features. The non-Poisson character of the source does not permit the use of the forward Kolmogorov equation or the Bartlette formula, two commonly used techniques in traditional noise theory. The method used in these papers was a probability-generating function combined with the linear character of the reactor noise in zero-power systems. In this paper we develop the Langevin approach to reactor noise in ADSs. Apart from being simpler, the Langevin approach allows treatment of feedback effects arising in ADSs with significant power as well as other noise sources, if any. We demonstrate that it is possible to obtain the correct expressions for various noise descriptors using this approach. The method is then applied to treat correlated non-Poisson pulsed sources with a finite pulse width including delayed neutrons. The present paper complements and expands our earlier discussions of ADS reactor noise.