FO OF A B737 HELD ON THE WRONG RADIAL IN A HOLDING PATTERN WHEN TROUBLESHOOTING AN ACFT UNUSUAL NOISE PROB. ATC INTERVENED AND ADVISED THAT THERE WAS NO CONFLICT CAUSED.

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ON LIFTOFF FROM LAS; WE GOT A LOUD BANGING NOISE UNDER THE COCKPIT FLOOR ACCOMPANIED BY A RUSH OF AIR. SINCE MAINT HAD JUST WORKED ON ACFT I ASSUMED IT WAS RELATED. NOISE FINALLY STOPPED AFTER 2 MINS. I TOLD ATC WE NEEDED TO LEVEL OFF FOR A POSSIBLE TURN BACK TO LAS. LEVELED AT 7000 FT FOR VECTOR. FO FLYING. I TOLD FO TO FLY ACFT AND I WOULD CALL MAINT AND DISPATCH. ALSO; I BRIEFED FLT ATTENDANT AND PAX. I ALSO TRIED TO MONITOR ATC'S INSTRUCTIONS TO FO WHILE I PERFORMED THESE OTHER DUTIES. I HEARD HIM ACKNOWLEDGE HOLDING INSTRUCTIONS ON BLD 210 DEG RADIAL; 10 MI LEGS. WE WERE IN EFIS ACFT AND I SAW HIM TYPE INTO FMC HOLDING INSTRUCTIONS. I FINALLY HAD TO LEAVE ATC FREQ ALTOGETHER BECAUSE I COULDN'T CARRY ON NECESSARY CONVERSATIONS AND LISTEN TO HIM TALK TO ATC AT SAME TIME. FO IS TYPED IN ACFT BUT A 6 MONTH FO. HE TYPED 210 DEGS AS INBOUND COURSE. HE SHOULD HAVE TYPED 030 DEGS. WHEN I CAME BACK ON FREQ I HEARD ATC SAY NO PROB; AND GIVE HIM A VECTOR. I ASKED WHAT THE PROB WAS AND HE SAID SOME CONFUSION OVER HOLDING PATTERN. THE NEXT DAY HE TOLD ME HE FIGURED OUT HE MUST HAVE HELD ON WRONG RADIAL. GOOD THING IT WAS XA30 AND LAS WAS LNDG ON RWY 1R AND NOT RWY 25 OR WE WOULD HAVE GOT TCASII ON ARRIVING TFC. I PROBABLY SHOULD HAVE MONITORED HIM CLOSER. THE NOISE WAS EQUIP COOLING FAN VALVE SLAMMING BACK AND FORTH. MAINT REPLACED VALVE.