I looked at the pic you have unfinished cab, can visualize where the cabinets above pass/driver are and how the screws help... did the bottom of the cabinets have a lining or were they bare?
On your oven rattle if it comes back don't forget to look at where I replaced part of the inside mounting boards with some padding on my replacement project.
sparky
My cabinets are finished inside. A year or so after we bought our PC, I added rubber shelf liner to the insides for noise and protection, but some prior abrasion occurred, especially in the other cabinet. I am ashamed to take a picture of that one. I was thinking of trying a little cherry stain there.
I thought of your idea with my microwave rattle, but didn't have to go there after tightening the six front facing perimeter screws. If it quickly returns, I'll be applying your padding idea. Thanks for that tip.
I always wondered how PC-USA attached the cabinets inside the B+ cap area. I still don't know how it was done. My 4 per side screws (bolts

) seem to have achored them down much better. I only wished the cabinet floor was thicker wood. I think it's just 1/4" finished plywood so there is still some flexing action. Just not nearly as much..