What was found
CABIN NOT PRESSURIZING. ON TAKEOFF, WHEN TAKEOFF POWER WAS APPLIED, CREW NOTICED A DEMONSTRATABLE BUMP IN PRESSURIZATION. IN THE CLIMB, CREW NOTICED CABIN PRESSURIZATION WAS CLIMBING OFF SCHEDULE. CABIN ALTITUDE CLIMBED AS AIRCRAFT DID. CREW STOPPED CABIN ALTITUDE FROM CLIMBING ABOVE 10,000 FEET. MASKS NEVER DEPLOYED. EMERGENCY WAS NOT DECLARED. WE DID NOT LAND OVERWEIGHT. CORRECTIVE ACTION: FOUND DUCTING IN TAIL CONE AREA LOOSE. SECURED PER CITATION 560XL AMM 21-21-00, / LEAK CHECK AND FUNCTIONAL CHECK GOOD.