What was found
DESCENDING THROUGH 3500 FEET, LOUD CABIN, HISSING SOUND STARTED FROM MAIN CABIN DOOR. FLIGHT ATTENDANT SAID NOISE WAS VERY VERY LOUD NO INITIAL INDICATIONS ON RATES OR ALTITUDE OSCILLATIONS ON BASE LEG AT 2000 FT, NOTICED RATE WAS DESCENDING AT 700 FPM APPROX. THEN CABIN CAUGHT UP WITH THE ALTITUDE & DURING APPROACH, CABIN FOLLOWED, DECENT RATE & MATCHED AIRPLANE DECENT RATE. LOST CABIN STARTING AT 3500 FT. HISSING STARTED ABOUT 3500 FT AND STOPPED AT APPROX 2500 FT, VERY LOUD & COCKPIT AND CABIN. REMOVED PASSENGER DOOR SEAL IAW CL650 AMM TASK 52-11-28-000-801. INSTALLED NEW PASSENGER DOOR SEAL IAW AMM 52-11-28-400-801. PERFORMED OPS TEST OF THE DOOR SEAL IAW AMM 52-11-28-710-801. NO LEAKS NOTED.