On May 26, 6:24 pm, wrote:

The microwave has been working normally for almost 5 years, but has
recently started making a buzzing sound on start up.

Sounds like your microwave is the culprit and ready to die. GFCI
circuits must be fairly sensitive to sense small inbalances in current
in their respective circuit. If the microwave is injecting
significant EMI back into your house circuit, no telling what could
happen. You could test this theory if you have some sort of emi
filter you could plug the microwave into between the outlet and oven.
I don't know if a surge protector power strip provides any sort of emi
filtering or if it just protects against over voltage spikes.