it may be the low voltage transformer
but it sounds like one of the door switches is also bad, there are also relays on the smart board that could be buzzing, maybe the one for the lights underneath, its really hard to say. the one thing you don't want to do is go pocking around in the control panel area, this can be extremely dangerous, here is the warning on this.
Consumer Product Safety Commission
Electrocution Hazard with Do-It-Yourself Repairs of Microwave Ovens
CPSC Document #5061
The U. S. Consumer Product Safety Commission (CPSC) warns against do-it-yourself repairs of microwave ovens. Each year, on average, four people die from electrocution trying to repair their microwave ovens. Microwave ovens use high voltage which makes it particularly hazardous for consumers to remove the cover and touch electrical parts. The possibility for electrical shock from a microwave oven still exists even after the oven is disconnected from the power source. Should certain internal devices fail, even unplugged ovens can cause serious injury.
The Commission strongly recommends that consumers have professionals repair microwave ovens because electrical appliances can be hazardous if consumers expose live electrical parts.
Removing the cover of your microwave oven and touching an electrical part can kill you.
Have a professional repair your microvave oven
its best to unplug this and call a tech. since we dont know what the issue is you dont want to take any chances, if this overheats it could catch fire, sounds like the control board though, this can be expensive to repair, wish i could do more , sorry, thanks Bryan