What was found
WHILE CLIMBING OUT OF DC AND PASSING 10,000 FT, THE F/A CALLED UP FRONT TO TELL US THE MAIN CABIN WAS MAKING A VERY LOUD NOISE. WHILE ON THE P/A WITH HER, IT WAS OBVIOUS THERE WAS A MAJOR PROMLEM WITH THE DOOR. THE NOISE WE HEARD WAS VERY LOUD AND MADE IT VERY DIFFICULT TO TALK TO HER. WE STOPPED THE CLIMB AT ABOUT 12,000 FT, REQUESTED LOWER ALTITUDE AND RETURN TO THE FIELD. UPON FURTHER INSPECTION BY MAINTANCE, THEY FOUND THE DOOR SEAL TO BE BAD. REMOVED AND REPLACED MAIN DOOR RUBBER TRIM SEAL.