I wonder if it is mainly the coils creating the sound rather than the entire motor.   After reading this article: http://www.jimhendershot.com/Jim_Hendershot/Articles_files/causes%26sources of audible noise in elec motors.pdf and browsing over these: https://books.google.ca/books?id=SUVuBwAAQBAJ&pg=PA308&lpg=PA308&dq=dampening+coil+noise+electric+motor&source=bl&ots=D6M4Kz3uL3&sig=Y4tyuSNH_aP8zUMhl_hRexcI3Yc&hl=en&sa=X&ved=0ahUKEwjKjN2xv8nUAhVjx