What was found
DURING CLIMB OUT, PASSING THROUGH 3000FT FLIGHT ATTENDANT CALLED ABOUT A VERY LOUD SQUEAL WAS COMING FROM MIDDLE OF CABIN, AROUND ROW 10. WE HEARD THE SQUEAL THROUGH THE INTERPHONE. WHEN WE LEVELED OFF AT 10000 FT SQUEAL STOPPED, PLANE WAS PRESSURIZING NORMALLY, WE RETURNED & SQUEAL STARTED AGAIN DURING TAXI & STOPPED WHEN PARKED. R & R NR 1 PACK RECIRCULATION SYS MUFFLER. OPERATIONS CHECKED GOOD IAW 21-24-02.