he observation of voltage-flux oscillations due to the radio-frequency–superconducting quantum interference device s rf-SQUID d effect in polycrystalline bulk MgB₂ superconductor is reported. This provides a direct evidence of the presence of Josephson junction-type weak-link grain boundaries in MgB₂. Analysis of the SQUID modulation indicates that only a few grain boundaries are of weak-link type whereas others are strong links. Generation of even and odd harmonics are observed when the MgB₂ sample is subjected to both ac and dc fields. This also indicates the presence of weak-link grain boundaries. Flux noise in the bulk rf-SQUID in MgB₂ is studied and compared with the flux noise in bulk high-Tc  rf-SQUID.