That is normal. All E2 two speed Rainbows make that sound. It is because it has a brushless motor that the computer is controling, and the computer is controling everything. The motor rotor has no coils on it on the rotor, which is very small and light, and the computer gives it a small power boost when you first turn on, then increases the power until ir reaches full speed.

If you have any more questions let me know. I am a Rainbow seller, and a Rainbow expert also.

Thank you,
Ralph