As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURN TO GATE FOR PASSENGER CONCERN OF SURGING SOUND UNDER FLOOR AT MID CABIN AREA. REPLACED MODULATING AIR VALVE, NO HELP. REPLACED CONTROL PRESSURE SWITCH, NO HELP. FOUND FILTER BYPASS POPPED. REPLACED FILTER AND ADP, LEAK CHECK GOOD. OPS CHECK NORMAL.