What was found
ON CLIMBOUT, EXPERIENCED LOUD FLAPPING NOISE BELOW COCKPIT FLOOR, UNABLE TO MAINTAIN PRESSURIZATION. CLOSED RT FLOW CONTROL VALVE AND NOISE CEASED, STILL UNABLE TO MAINTAIN CABIN PRESSURE. RETURNED TO SJU WITHOUT INCIDENT. MECHANIC FOUND LOOSE CLAMP AT RT FLOW CONTROL DUCT AND SECURED SAME. OPS CHECK IAW NASI MM, CAUSE UNDER INVESTIGATION.