[FONT=Times New Roman][SIZE=3][COLOR=#000000]The main board will slow and speed up the motor, as needed depending on thermistor readings and different things. However it should take one minute for the motor to change speeds and most of this time this is unnoticeable. Your fan is changing speeds rapidly which is caused by a bad main board. [/COLOR][/SIZE][/FONT]
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[SIZE=3][COLOR=#000000][FONT=Times New Roman]This is a fairly common issue with these models. [/FONT][/COLOR][/SIZE]