Measurement of reactivity in deep sub critical system using neutron noise method is challenging due to the modal effect. This paper addresses the issue of modal effect in such system in a new and better way. The measurements were carried out in the ADS experimental facility BRAHMMA commissioned recently at Bhabha Atomic Research Centre (BARC), India. The BRAHMMA assembly is a deep sub critical system having multiplication factor (K_eff)~0.9. The data analysis was carried out using two commonly used neutron noise techniques namely the Feynman alpha and the auto correlation function methods. The modal effect was eliminated by placing the neutron detectors at strategically chosen locations in the core. The detector locations were decided on the basis of the detailed modal analysis of the core. The modal analysis and calculations of the kinetic parameters of the core were carried out using a diffusion theory code.