I'm also getting this problem, its very frustrating, i have taken the airmanager to bits and it can be only one of 2 things, 1-PCB 2-The fan itself. Both of these look completely fine to me and they are less than 2 years old, Looking at the fan makers website they guarantee their fans for 6 years... is Ultimaker going to help its customers here?
I currently have my fan uninstalled from the machine and control screen.