@SweetSydney wrote:
Update: Trees have started having a humming noise. 
I'm going to continue running them & see if the humming noise gets worse. The two Silver trees I returned didn't have a humming noise. That was one of the complaints people had in their reviews as to why they returned the trees.
I'm really hoping the humming noise doesn't get worse.
@SweetSydney - For whatever it's worth, two of my three trees have a bit of a humming noise from the motor that I can hear if I'm in the room with them; I can only hear the noise from the other one if I'm quite close to it. Overall, the sound is still very quiet, though, and the humming seems to ebb and flow a bit over the course of the time the trees are running. Despite any claims to the contrary, I've never encountered a completely silent motor in any mechanical piece.
Basically, I wouldn't worry that the motor sound means that there's anything wrong with your trees; I guess it just depends on how much it bothers you.