LOUD EXTERNAL AIR NOISES AND INAPPROPRIATE EICAS MSGS ALERT CRJ FLT CREW. AFTER LNDG; NOISES ARE TRACED TO AN O2 SERVICE DOOR LEFT OPEN BY GND CREW. FLT RETURNED TO DEP ARPT FOR MAINT.

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UPON TKOF FROM ZZZ; WE IMMEDIATELY RECEIVED AN AMBER APU FAULT AND APU LOW PRESSURE INDICATION ON THE EICAS. AT THE SAME TIME; WE NOTICED A LOUD SOUND COMING FROM THE FORWARD R SIDE OF THE ACFT. THE NOISE WAS LOUDER AND MORE PRONOUNCED THAN A GPU OR INTERCOM DOOR BEING LEFT OPEN. IT SOUNDED LIKE THE ADG HAD DEPLOYED AND WAS RUNNING. I SHUT DOWN THE APU ON THE CLBOUT. AFTER CLEANING UP THE ACFT I CHKED THE ELECTRICAL SYNOPTIC PAGE AND THE ADG WAS NOT SHOWN PWRING THE ESSENTIAL BUS. UPON THE CLBOUT WE ALSO RECEIVED THE 'STALL FAIL' AMBER MESSAGE. I MADE THE DECISION TO RETURN TO ZZZ. I DID NOT DECLARE AN EMER BUT DID HAVE THE TRUCKS STANDING BY DUE TO THE UNCERTAINTY OF THE NOISE UP FRONT. THE FLT ATTENDANT; MAINT; ATC AND THE PAX WERE NOTIFIED. WE LEVELED OFF AT 4000 FT AND PROCEEDED WITH A R PATTERN BACK TO RWY. I ASSUMED CTL OF THE ACFT ON BASE AFTER ALL PARTIES WERE NOTIFIED AND WE WERE READY TO LAND. LANDED WITHOUT INCIDENT; TAXIED TO GATE AND NOTIFIED ATC THAT THE EMER TRUCKS WERE NO LONGER NEEDED. I CHOSE TO DE-BOARD THE ACFT BEING WE HAD NOT RESTARTED THE APU AND THE R ENG WAS STILL RUNNING; POWERING THE ACFT. UPON INSPECTION; THE OXYGEN DOOR WAS OPEN AND NO OTHER SIGNS OF ABNORMALITY WERE OBSERVED.