Why does the LG Microwave range hood hum loudly when the fan turns on

We had a similar odd sound from the back of our refrigerator.

There's a few different parts back there that can make noise: The compressor, condenser fans, and defrost timer. Apparently the fans can be cleaned fairly easily, and the timer can be replaced if its electric motor starts making noise.

In my case, we called in an independent service technician (found via Service Magic). He diagnosed the problem for us at no charge. Unfortunately for us it was the compressor, and a new unit would cost almost as much as a new fridge, so we ended up getting a new fridge.

But in your case, maybe you're lucky and it's one of the other parts.

It is (very) surprising that the gas company (Xcel) says "everything's fine." This document from a different gas company certainly calls out non-blue flames as an issue that needs to be resolved. As does this site. And Xcel themselves.

Happening on all appliances does seem a bit less likely to be "and suddenly the air adjustments on all three went whacky" than a contamination issue - possibly water in the gas line?

I wonder how your furnace tech visit went, and what your neighbor's flames look like.

Editing post-comments: Ultrasonic humidifiers make small water droplets (without evaporating the water first) and then those small droplets evaporate while floating around - if the water contains salts or minerals, any salts or minerals in the water will end up floating around the house as tiny specks when the water evaporates from the tiny droplets. Those, evidently, are what is contaminating your gas flames as they are sucked in along with the air required to burn the gas. Old-style "sponge" dehumdifiers (should) only result in water vapor in the air, as they actually evaporate the water (and this eventually leads to a build-up of scale and salts in the humidifier and its sponge element, as those do not evaporate.)