For about 1 month my 2011 A6’s fresh air blower has been going crazy, at first it just stopped working, no air coming out at any speed, but then after i parked it up overnight it was working again the next day. Since then it has just been out of control, it will work for a few minutes and the speed will fluctuate constantly. 
 

i am just wondering if anyone is familiar with the humming sound that it is creating? It’s audible in the cabin and also the engine bay, noise disappears when i turn off the climate control. You have to tune your ears out from the agricultural sound to hear it in the engine bay 
 

I plan on swapping out the blower unit this weekend, but just want to know if there is anything else that i should be looking for whilst doing so? 
 

any help appreciated. 

Check the air quality sensor under the wind screen