My heater / AC fan just started buzzing kinda like when you would put a baseball card in the spokes of your bike wheel. The AC/ heater system is also making a strange flopping noise when you turn back and forth from the AC setting. It didn't make a flopping thump sound before the buzzing started. The transition between AC and the other settings was smooth.

I'm afraid something in there has broken loose and is now interfering with the fan. I searched this forum but found nothing to guide me.

Please help.

 

Sounds like you may have a leaf or two that have made their way thru the air intake grill on the wiper cowl and is lodged againt the blower wheel. It happens!