Hum no smell
okay that can be only one of two things. 1 the magnetron. appliancepartsimages.com/dbImages/i/00002866/Magnetron-WB27X10017--00862204.jpg" alt="Magnetron WB27X10017 Alternate Product View" width="100" height="78"/> $189.00 or 2
the high voltage transformer. Transformer-WB27X10133-00718458.jpg" alt="High Voltage Transformer - Part # XXXXX Mfg Part # XXXXX"/> $205.00 , so at this point you need to make a decision, repair or replace. I wish it was the diode truly, sorry, Most people choose to replace the microwave and theres good reason for that, if its the transformer it may have already damaged the magnetron and that could fail at any time. the only way to know which one is bad is to remove one wire from the magnetron and isolate it start the microwave and if you still have the loud hum it the transformer. be careful if you decide to do this. the high voltage capacitor store lethal amounts of voltage.