I figured this was probably the magnetron -- And a little reading on line reinforced that. Ordered replacement magnetron and diode, as well as door torsion springs on a separate order (might as well get it all done while the box is open).
While there are a lot of pieces to remove and screws to keep track of, the repair was simple and didn't take long. Just be sure you discharge your capacitor so you don't light your head on fire.