ACR MLG IN CRUISE HAS NMAC WITH UNKNOWN ACFT.

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WE WERE INBND TO DET METRO FROM THE SE. WERE CLRED TO FL240 TO CROSS DJB AT FL240. THEN WERE CLRED TO CROSS TRIPE INTXN AT 10000'. JUST AS WE WERE ABOUT TO BEGIN THE DSCNT; WE BOTH HEARD A LOW; MOANING VIBRATION. HAVING NO OTHER INDICATIONS; WE ASSUMED THAT IT WAS IN THE ACFT AND SCANNED ALL THE INSTRUMENTS AND WARNING LIGHTS FOR PROBS. NOTHING WAS FOUND; SO WE ASSUMED THAT IT WAS THE NOSE GEAR DOORS AND TRIED THE OLD REMEDY OF TURNING THE HYD PUMPS TOO HIGH. NO EFFECT WAS NOTICED; AS THE NOISE HAD LONG BEFORE CEASED. WE WERE ABOUT TO DISMISS THE INCIDENT WHEN ONE OF THE F/A'S CAME FORWARD AND ASKED; 'DID YOU SEE THAT?' WE; OF COURSE; ASKED; 'SEE WHAT?' SHE THEN DESCRIBED A SMALL HIGH WINGED AIRPLANE THAT SHE HAD SEEN OUT OF THE LEFT SIDE OF THE AIRPLANE. SHE FURTHER SAID THAT IT HAD BEEN VERY CLOSE. FURTHER INQUIRY REVEALED THAT THE NOISE WAS MUCH LOUDER IN THE CABIN AREA. WE CONCLUDED THAT WE HAD JUST HAD A NEAR MISS. THE AIRPLANE HAD BEEN SO CLOSE THAT WE HEARD IT! IT HAD EVIDENTLY PASSED RIGHT TO LEFT OVER US ON A VERY SMALL HDG XING ANGLE; AS WHEN THE SOUND WAS HEARD; I WAS LOOKING DIRECTLY AT THE 3 O'CLOCK AREA. CLE CENTER WAS ADVISED; BUT HAD NO PRIMARY TARGETS NEAR US.