We characterize the influence of interfacial trap sites on carrier scattering and subsequent contribution to channel noise by taking 1/f noise measurements on pentacene organic field effect transistors (OFETs). The noise dependence on drain current from OFETs with UV-ozone treated parylene gate dielectric before the deposition of the semiconductor is compared to that of otherwise identical OFETs with no air exposure during fabrication. Our studies indicate a different noise characteristic in the two samples, which is further confirmed by increasing the carrier density under illumination and comparing the noise spectrum for photogenerated charges with gate-field-induced carriers.