Whistle from air vents

[wingz4]

I have been searching this for two days, and can't figure out which one is similar to my problem. My 2007 X5(E70) When I turn on my rear air conditioning after a while it makes some whistling noises but if I turn one level up or down it will go away for a little while but will eventually comes back again. Does anyone know how to solve this problem? Or can anyone give me the instructions on how to change the rear blower motor so i can take it out to clear it up change it?
Thank you so much