Hi, this is Bryan, normaly when a microwave does this it turns out to be the magnetron which is one of the most expensive parts of the machine also repairing these needs to be done by a technichian, this is what the goverment warning says.
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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.
/!\ WARNING ELECTROCUTION HAZARD HIGH VOLTAGE Removing the cover of your microwave oven and touching an electrical part can kill you. Have a professional repair your microvave oven
being 15 years old I would replace this, its more cost affective, thanks Bryan