Good idea to run at half power. Good thinking. That can sometimes reveal an overheating magnetron.
Does it seem to buzz or hum louder than normal?
If so, you may have a shorted high voltage diode or a bad magnetron. If not, I only suspect the magnetron.
If it's less than one year old, it should be fully covered under warranty.
I'd suggest contacting the manufacturer to get it fixed free.
it's within the magnetron portion of the warranty (usually 5-9 years)
but out of the original full warranty (usually 1 year), then your
manufacturer will cover the part cost, but not the cost to visit you or
the labor charge to do the work.
You can usually find helpful exploded view diagrams and order parts by entering your full model number here
We have critical safety & disassembly info (including a how-to video) at our site, linked here
should also be a "mini-manual" hidden inside the unit behind the
control panel or hidden on the left side behind the grille, which is
very helpful when troubleshooting & testing.
Whirlpool customer service is here
.We're happy to help you and we'd appreciate your
thoughtful rating of our answer.