I've found that most times bearing noise and vibration increase as the speed increases. The noise only occurring above 2100rpm suggests that it is smething other than bearings. This noise could be a characteristic of the machine. Another 704 owner or Grizz tech support may be able to advise. After talking to Grizzly, if you wanted to isolate the area the noise was coming from you could remove the motor so it no longer drives the spindle gears, secure it so it does not move and run it "uncoupled" at different speeds. If you still hear the noise it is motor related, if not it could be from the gears or the spindle bearings.
This machine would have to be pretty noisey for me to want to take it apart.
I recently purchased a Grizzly 720R Mill. At all speeds it makes an intermitent rumbling noise that increases in intensity as the speed increases. I think it is normal noise from the gears.
Mikail