A couple of things could be the problem;
1. A animal has crawled into the vent and died
2. The AC evaporator coil is full of dirt and mold.
My money is on #2.
Get some coil cleaner from a chemical supplier who sells janitorial products. Make sure that the drain tube to the AC is clear. You can very carefully make a hole in the housing of the AC and spray the cleaner into the system. Use a garden pressure sprayer rinse the coil with water to flush the debris out the drain. When you are done flushing out the coil use a rubber body plug to seal the hole. Run the system with the AC set to operate at the WARMEST setting ie; max AC, temp dial to hot. This is to dry out the coil area of water.