In industrial crystallization, understanding the effect of individual secondary nucleation mechanisms is difficult. This paper presents frequency response of a mixed suspension mixed product removal (MSMPR) crystallizer to a periodic modulation in feed rate. This will help in roughly understanding the effect of magma density or thick solute layer adsorbed on crystals, on the formation of secondary nucleation, which are otherwise practically impossible to understand through stirrer speed modulation (V. Bal and B. Peters, Cryst. Groth Des., 21 (2021) 227–234). Feed rate modulation drives the oscillation in magma density, which in turn drives the supersaturation. Combined effect of magma density and supersaturation controls the oscillation in nucleation rate and hence, damped oscillation in number density. In the intermediate frequency limit, strong oscillation in either amplitude or phase shift can be exploited to study the different secondary nucleation mechanisms. There is a narrow range of frequency and amplitude of feed rate modulation over which the crystallizer should be operated for detectable response.