I had the same problem. Whenever I used my microwave, it started with a loud and annoying rattle / buzz that tapered off over the first 2 or 3 seconds.
I took off the microwave's bottom panel and discovered that the internal frame behind the control panel was bumping / vibrating against the side of the microwave. So I stuffed a couple pieces of paperboard in that crack, and the noises have been drastically diminished ever since.
My microwave is an Amana Radarange (model ACO1860AS), over the stove type.
phuntism, September 2009