I had this a couple of months ago, the car was parked up for a week or so in the backyard.
I just replaced the cabin filter, deodorised the vents and was all good.
It was a mouse and it had started to remove bits of the sound deadening from behind the trim panels and was setting up a nest in the blower motor.
It was still in there when I was cleaning it out and my Border Collie Garry nabbed it when it legged it from somewhere in the engine bay haha