Maybe double check that all your bulbs are dimmable. The only time I've run into that buzzing is when I have a bulb that isn't dimmable. Alternately, you might have a bulb that's going bad and not dimming correctly.

We did a couple poly coats on our cabinets after painting. Just make sure to let the paint fully cure before doing so. Look at the manufacturer spec sheet for that as I recall it was longer than I would have guessed. The added durability seemed worth it to us.

Oh, and we used General Finishes water based poly. They had good videos on the application process.