I replaced the magnetron, the buzzing continued unabated.
We generally use the appliance for short periods, warm up items, coffee, tea, etc. The buzzing is fairly loud during these events, recently I had the unit on for a longer period and the buzzing stopped. I find that whenever the unit is on for longer than 3 minutes there is no buzzing.
I am guessing the problem is really the xfmr, high current at 60 hz is probably shaking the darn thing, it probably heats up sufficiently by 3 minutes, expanding the wiring preventing the oscillation. Anyhow, replacing the xfmr isn't a cost effective solution given the current price of a new unit.
Thanks to everyone for their suggestions and help.