After checking on the internet for the probable causes of the noise in my microwave and confirming the diagnosis by listening to where the noise was coming from, I decided to replace the magnetron. A few days later, the part from AppliancePartsPros.com was delivered to my door.
I've never repaired a microwave before, but it wasn't too difficult; I just kept removing parts until I could get to the one I was looking for.
After buttoning everything back up, the oven worked perfectly. And only 2 screws left over!