RE: Digital or Coreless...
Which model of digital did you get?
Not all digitals buzz or sing, it's generally only the high-performance ones. I have some some of Hitec's entry-level digitals (HS5475) and they don't buzz, neither do the Futaba S3151 units. My coreless HS5925 digitals however, sing like a bunch of banshees :-)
Why did you pull your servo apart? There's a lot of wisdom in the old adage: "if it works, don't fix it."