B737-300 FLC RPT OF A POSSIBLE NMAC OR POTENTIAL CONFLICT WITH AN E2 ACFT THAT SHOWED 1000 FT ABOVE ON TCASII AS IT PASSED OVERHEAD. THE ENG OR ACFT NOISE WAS TOO LOUD FOR THEM TO BELIEVE THAT THE ACFT WAS THAT FAR AWAY.

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WE WERE AT CRUISE AT FL250 ABOUT 50 MI S OF SWL VOR. OUR DIRECTION WAS NBOUND IN SOLID IMC. WE NOTICED A TARGET APCHING US ON TCASII IN A DSCNT. AS THE TARGET MERGED WITH US I FELT A VIBRATION AS WELL AS VERY LOUD ACFT ENG NOISE. THE TCASII INDICATED TARGET WAS 1000 FT ABOVE; BUT THE NOISE AND VIBRATION I FELT WOULD INDICATE TO ME THAT THE OTHER ACFT WAS MUCH CLOSER. I'M GUESSING; BUT I WOULD SAY WITH THE NOISE LEVEL; THE OTHER ACFT PROBABLY PASSED WITHIN 200-400 FT. THE FLT ATTENDANTS ALSO HEARD THE ACFT PASS BY AND CAME RUSHING UP TO THE COCKPIT TO FIND OUT WHAT WAS GOING ON. IN MY YRS OF FLYING; I HAVE NEVER HEARD THAT LOUD A NOISE BY AN ACFT THAT WAS PASSING 1000 FT ABOVE ME. SUPPLEMENTAL INFO FROM ACN 363706: TARGET GIVES A BRIEF TA ON TCASII; TARGET SHOWS '+10' ON DISPALY AT 2 MI. WHEN TARGET MERGES WITH ACFT SYMBOL ON OUR TCASII DISPLAY; BOTH CAPT AND FO HEARD A LOUD 'VROOOM' SOUND PASS BY (IMC) THAT SOUNDED LIKE A TURBOPROP ACFT. FLT ATTENDANTS RUSH TO COCKPIT TO ASK IF WE HAD JUST PASSED BY AN AIRPLANE! WE HAD ZERO VISIBILITY. OTHER ACFT SAYS HE HAD US IN SIGHT!