GE Profile MW doesn't heat and makes a humming noise

Hi - our GE Profile Spacemaker II microwave just stopped heating (otherwise works fine) and is making an odd extra humming sound. I’m fairly handy and have a meter if needed. What’s the likely culprit? I’ve seen some posts about bad magnetrons and think it’s under warranty (parts at least).

Thanks!

… and threatening to buy a new one if I can’t fix it soon! thanks