I have a 2 year old LG LMVM2277ST microwave that is developing a loud hum on startup. Currently the problem is interrmittent, but frequent, and subsides to normal after a few seconds. I received a reply asking if the microwave still heats, (of course it does, I didn't state it didn't) and it was given three major components, the rectifier, the capacitor and the magnetron. But the response was for a non- heating unit. Not my scenario. Can you tell me the most likely cause of a loud hum on startup in a working microwave? Thank you

The hum you're hearing is mechanical. There is a large transformer that provides the voltages for the magnetron tube, and it's normal for a transformer to hum to some degree. It's because of how they are built, caused by vibration of the iron pieces used in the transformer core. Glue is used in their construction to help deaden the hum, and over time it can lose its effectiveness and the hum is more noticeable.

The magnetron has a large has a large initial current draw when it first comes on, and the transformer has to work harder which can make the hum louder. Then the current drops and the hum decreases. Some brands (Sharp, particularly) had hums so loud that the transformers required replacement. As long as the oven appears to operating normally otherwise, keep on using it. It's annoying but not hurting anything.

It is possible that the hum is louder in general because the magnetron is just working harder. As it ages, a magnetron's power drops and it draws more current to keep going. This makes the hum louder. Two years may be rather soon, but if the oven's seen lots of use that might be part of the trouble. Again, as long as the thing is cooking, keep using it. Sooner or later the magnetron will go bad and the hum will be obnoxiously loud. That is, it will be if the magnetron shorts out when it goes bad; it can also open circuit, which will cause the hum to drop off and there will be no heating.

I hope this clears things up. Thanks for using FIxya!