What was found
(CAN) DURING CRUISE FLIGHT, THE CREW SELECTED CROSSFEED TO CORRECT A FUEL IMBALANCE. TWENTY SECONDS LATER THE CREW HEARD A RUMBLE WHICH THEY SAID SOUNDED LIKE A COMPRESSOR STALL AND RETURNED FOR LANDING. NO ANOMALIES WERE NOTED ON THE ENGINE INSTRUMENTS. MAINTENANCE DETERMINED THAT THE ENGINE NOISE HAD BEEN CAUSED BY TRAPPED AIR IN THE CROSSFEED LINE. THE CROSSFEED LINE HAD BEEN REMOVED AND REINSTALLED PRIOR TO THE FLIGHT IN ORDER TO FACILITATE REPLACEMENT OF THE OUTFLOW VALVES. THE INTERRUPTION OF THE FUEL FLOW WAS CONFIRMED BY FDR DOWNLOAD. THE MAINTENANCE MANUAL MAKES NO MENTION OF BLEEDING THE CROSSFEED LINE AFTER REPLACEMENT.