After nine years, I think I resolved the loosening and then rattling of the microwave oven, thanks to this idea from my cousin Richard.
I removed the six screws around the perimeter and pulled out the microwave oven.
I drilled the top two and middle two holes in the cabinet bigger to accept spiked "T" nuts, the kind often used in furniture, and used threaded screws and lock washers to replace the four wood screws. I replaced the bottom two wood screws with other wood screws that cosmetically match the upper four. All hardware used as shown.
Here is one spiked "T" nut pressed into the enlarged hole from the inside of the cabinet. The hole in the "T" nut is threaded.
I added thick felt pads on the plastic rear feet and thin felt pads on the front rubber feet as not to change the height in front.
I reinstalled the microwave and was then able to tighten down hard the top 4 screws, much tighter than before when they were wood screws. The bottom two screws had to remain wood screws because there was no means to insert the "T" nuts.
Hopefully the rattling is over for good. Previously, I had to occasionally tighten up the six wood screws which helped for a while but they would loosened up again and then came the rattling. I just could not tighten them enough without stripping them.