2001 Hyundai Tiburon started to smell like dead rat

Check for anything that may have spilled, or have died inside or under the hood. Car was fine until two days ago. Just a usual trip out of town, next day, just a terrible odor, like mildew, and something rotting.

trying to clean using A/C duct cleaner, but not too successful. Any ideas why the odor so sudden? And how to remove it?

thank you!