I don't think it's mice.

Try moving the handle to adjust the angle of your ceiling register grilles. In my situation, the noise only came out during windy days, either day or night. Normally, you won't hear mice activities during daytime. But if they do find their ways to get into your attic, you must be able to see droppings in your attic, and they poop constantly. They will not keep your attic clean and go directly to your A/C duct.