What was found
ON TAKEOFF PRESSURIZATION WAS NORMAL UNTIL APPROXIMATELY 500 FT WHEN LOUD RUMBLE OCCURRED AND PRESSURIZATION BEGAN =- 500 - 1000 FPM RATE OF CLIMB AND DESCENT. AIRCRAFT RETURNED TO ABQ. NOISE AND RUMBLE REMAINED UNTIL PRESS RESET FOR LANDING. INSPECTED A/C DOORS AND SEALS, OUTFLOW VALVE AND LUBRICATED EQUIPMENT COOLING FAN FLOW CONTROL VALVE. PERFORMED PRESSURIZATION FUNCTION CHECK, OPS CHECK NORMAL.