A randomly packed bed of nearly uniform spherical particles is employed to study cocurrent air/water downflow. Using a high-speed video camera, observations are made in the interstices close to the bed wall. Patterns in the microscale (in constrictions and pores), similar to those recently reported for a two-dimensional test section, combine to yield macroscopically observed flow regimes. A gas penetration mechanism is identified as a dominant feature at incipient as well as in well developed pulsing flow. Special attention is paid to pulse formation and growth. A new, nonintrusive conductance technique appears to hold distinct advantages, over other methods, for on-line detecting the onset of the pulsing regime. Time records obtained with this technique complement information from flow visualization, shedding light on the mechanism of transition. Measurements of the trickling/pulsing transition boundary are in fair agreement with other data and model predictions from the literature.