Flight Crew Air Transport Pilot (ATP)
Communication Breakdown
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed
Aircraft; Human Factors; Environment - Non Weather Related
During the preflight inspection; we noticed in the logbook a couple of maintenance items concerning a noise emanating from the main cabin door during flight.  The door and its seal had been inspected by Maintenance and a leak check had been conducted; with 'ops checked good' (or words to that effect) in the corrective action.  If memory serves; the most recent write up described the above noise as a 'hum.' We departed and climbed away from ZZZ uneventfully.  While climbing through FL210 at a cabin PSID of 7.5 we heard the noise from the cabin door.  It was not a hum; rather a loud shout.  The Flight Attendant later described it as sounding like a 'chain saw.'  Indeed; when the forward Flight Attendant went to call us; he had to go to the back of the aircraft and use the aft interphone as it was virtually impossible to communicate with that racket going on.  We asked ATC for a level off and then a decent to try to make the noise less with a lower PSID.  A quick check of the cabin pressure and PSID revealed no issues there; the cabin pressure was working normally.  Indeed; checking the cabin alt vs aircraft alt in the QRH's manual pressurization schedule revealed that everything was fine.  I tried calling Maintenance in ZZZ as we should have still been within radio range; but I think that they may have gone home for the night.  I also ACARSed Dispatch and then called Maintenance Control on ARINC.  It took a while to get through to them due to COVID-19.  When I appraised them of the situation; they told us to return to ZZZ.  I talked to the flight attendants and talked the passengers to let them know what the situation was.  It was a bit frustrating for the latter as the roar from the door was a soft hum aft of row X.  To add insult to injury; a thunderstorm developed over ZZZ in our absence; so we had to hold...which was fine as Dispatch still had to round up operations agents; gate supervisors; ramp agents; fullers etc.  To add insult to injury; after we landed; the ramp was closed due to lightning.  After we blocked; I gave Maintenance a quick shout on the telephone and we offloaded the passengers and made our way over to the next aircraft for the flight to ZZZ1.  Changing the subject; I would mention that the FO (PF) did a fine job flying the aircraft and dealing with ATC whilst I was talking to the Maintenance/Dispatch/flight attendants/passengers and to the FAs for taking care of the passengers.Apparently there is something afoot with the door seal on aircraft XYZ.  I guess Maintenance's efforts as to try to rectify any hums/roars/squeals etc hadn't solved the problem.
Air carrier Captain reported a loud hum during climbout emanating from the door and having to return to departure airport.