It sounds like a woodpecker.....

Hi, I am newcomer here and need some help. My '93 K2500 has an odd thing. Every time I turn on the ignition I have this woodpecker sound coming from in front of the heater/AC fan on the passenger side. I felt underneath and can feel a little flapper moving. I think it is the air diverter that has a solenoid or limit switch that is out of adjustment. How hard is it to remove the fan to get to that item? Thanks for any help.