Using the A/C with fresh air produces a musty smell of the air entering the cabin from the HVAC system. I looked in the manual if there was an air filter for the cabin to change with no avail. After returning from an overseas military assignment where the truck was in storage for 3 years, took it for service and asked the dealership to troubleshoot and solve the problem. There was no filter to change/replace so the service department flushed the fresh air intake (at the base of the windshield with a pressure washer/cleaner) and also ensured there was no clog for the A/C drain. Now the fresh air and A/C does not have the musty smell.