A/C is turned on, there is a fluttering noise that comes from the cabin filter area?

You will want to pull the blower motor and check for debri. Chances are good that when the cabin filter was replaced some debri fell down into the blower. I see this all the time at work. Also you may find a lot of debri in there or a nest from rodents if this is the case then you will want to put the heater on recirculate before shutting the vehicle off. This will slow the rodents from getting in there. If that is okay chances are you have a blend door actuator that is making the noise.