My microwave is not heating, and now is making a loud noise if turned on. I’ve read it could be the magnetron? It is recommended that I call for service. A sevice call is min $79 plus parts…is it better to think about new unit? This one cost about $350 or should I have sevice appt to diagnose then repair myself??? 
Besides the magnetron it could be the diode or the high voltage transformer (very unusual).
If the microwave is less then 5 years old then the magnetron is under the limited (part only) manufacturer warranty.
It’s hard to tell you which way to chose.
Gene.