I found other complaints of the same problem on the internet, especially this site, and the diagnosis. The motor sound turned out to only be the interior fan running - no radiation.The parts diagram showed 2 door switches (Part #AP2024337) and 1 switch monitor ((Part #AP2024338) located behind the control panel as the possible source of the problem. A YouTube video showed how to discover the faulty part and replace it. It was SUPER EASY, taking less than 20 minutes to fix, and only cost $20 (shipping included) and 1 day for the part to arrive from ApplianceParts Pros.com with regular shipping. I only had to remove 5 screws total, using a Phillips screwdriver and a torx (star-tipped) screwdriver. (Unplug the power, first.) I am thrilled I didn't have to run out and spend major money for a new microwave and am grateful to AppliancePartsPros for their online information and easy order service.