Microwave humming when the door is closed

The other day I attempted to heat up some food in my microwave and after a few cycles I was noticing it was not heating the food up as it used to. Still worked but seemed weak. When I set a power level then start my food it does heat up. Now when the door is closed I get a humming sound. I looked it up and most things point to the magnetron. I am not too experienced with appliance issues so I wanted make sure this was my issue before ordering the part.

I just got back home and currently it is no longer humming but now it will not heat up. I tried setting a powerlevel as I did last night but it did not work this time either.