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Unit uses a Magnetron and likely a power board called an inverter to run it. Someone would have to open the unit in a way where they might pinpoint where the sound is coming from and then take action--does not of course sound normal or good to me.
I have a 2003 Sears Ultra Wave microwave 721.63682300 with a similar problem. A few months ago it started giving off short buzzes when turned on. That stopped but then I noticed that everything took twice as long to heat up. After quite a bit of teardown&inspection I found a circuit break in the inverter board leading to ZD704 diode (pictured). I will now try to find a replacement diode to see if replacing will fix it. Thanks for your postings.
The magnatron is shorting and overloading the high voltage transformer
which is complaining with a load noise. Worth fixing if you do it
your self. Magnatrons cast about $40 wholesale. There are
just a few varieties of them that fit almos all brands. Try
Although your model number doesn't seem to show up, the NNS950-series units are powered by an high-voltage inverter. And yours has failed.
While an inverter (switched-mode power supply) is more efficient from a power consumption standpoint, it is more complex, thus more statistically prone to fail. These should only be examined and worked on by someone who has the necessary technical qualifications. Seriously.
Often the machine-soldered connections in a microwave inverter are too weak, so they eventually fail, especially if heat is high in the area. They sometimes fail catastrophically.
When not catastrophically damaged, you may hear clicking or squealing and may smell an electrical odor, which is sometimes obscured by the cooling fan.
Sometimes a whole new inverter is cheaper than the time and labor required to fix the old one.
If a technician is willing and able to troubleshoot and repair the inverter to component level, it will may be expensive, but may be worth it, especially if he or she will resolder and reinforce connections as needed as part of a thorough repair. That could easily make your inverter beter than a new one!
You might be able to get some verbal estimates if you call Panasonic-authorized appliance / microwave repair shops and ask then specifically about inverter component-level repair and simple inverter replacement costs for your exact model number.