We study the preheating scenario in generalized Einstein theories, considering a class of such theories which are conformally equivalent to those of an extra field with a modified potential in the Einstein frame. Resonant creation of bosons from an oscillating inflaton has been studied before in the context of general relativity taking also into account the effect of metric perturbations in linearized gravity. As a natural generalization we include the dilatonic-Brans-Dicke field without any potential of its own and in particular we study the linear theory of perturbations including the metric perturbations in the longitudinal gauge. We show that there is an amplification of the perturbations in the dilaton-Brans-Dicke field on super-horizon scales (k-->0) due to the fluctuations in the metric, thus leading to an oscillating Newton's constant with very high frequency within the horizon and with growing amplitude outside the horizon. We briefly mention the entropy perturbations generated by such fluctuations and also the possibility to excite the Kaluza-Klein modes in the theories where the dilatonic-Brans-Dicke field is interpreted as a homogeneous field appearing due to the dimensional reduction from higher-dimensional theories.