Car Vents Smell

My 2011 Honda Accord has a moldy smell when the set to bring in fresh air from outside vents. The smell isn't as bad when set to recirculate. The fan doesn't even have to be turned on to get the smell, the small amount of air coming through the vents is enough to smell the odor.

I have replaced the cabin air filter and sprayed auto vent cleaners in all the vents, included the outside vents.

Any thoughts on what could be causing the issue?

Thank you!

dead mouse in the ducting?