A SAAB 340 CREW CHOSE TO RETURN LAND WHEN THEY FELT A STRANGE BUMP ON CLB OUT. NIGHT OP. WX FACTORS SOLID IMC.

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WE DEPARTED RWY 3C. AFTER TKOF WE WERE VECTORED BY DTW DEP TO INTERCEPT THE DTW 108 DEG RADIAL TO WINGS; ON COURSE. ATC CALLED OUT TFC AT OUR 10 O'CLOCK POS; A FALCON JET PASSING IN FRONT OF US; L-R; AND INFORMED US HE WOULD HAVE HIGHER WHEN WE WERE CLR OF TFC. WE CALLED THE TFC 'IN SIGHT' TO ATC AND RECEIVED A CLRNC TO 9000 FT. AFTER INTERCEPTING THE DTW 108 DEG RADIAL AND CLBING THROUGH APPROX 7000 FT; WE (CREW AND PAX) HEARD A LOUD THUD WHILE THE ACFT WAS DISPLACED VERTLY UPWARDS FROM ITS CLB PATH. THE NOISE SEEMED TO COME FROM THE BELLY OF THE ACFT ABOUT MIDSECTION AS JUDGED BY THE VIBRATIONS I FELT FROM THE FLOOR BOARDS. I QUICKLY ASKED THE FO TO GET THE FLT ATTENDANT UP INTO THE COCKPIT WHILE SIMULTANEOUSLY SCANNING THE INSTS FOR ANY ABNORMAL INDICATIONS. EVERYTHING LOOKED NORMAL AND THE ACFT DISPLAYED NO ADVERSE AERODYNAMIC EFFECTS. I CONFERRED WITH MY FO AND HE AGREED THAT IT FELT LIKE WE HIT SOMETHING. THE FLT ATTENDANT CAME UP FRONT AND I ASKED HER IF SHE HEARD AND FELT THE LOUD THUD AND JOLT. SHE AGREED THAT SHE DID; BUT WAS UNCERTAIN AS TO ITS LOCATION. MY FIRST THOUGHT WAS THAT OF A MIDAIR; BUT I QUICKLY DISMISSED THAT SINCE THE ACFT WAS BOTH FLYING AND PERFORMING NORMALLY AND THE ONLY RPTED TFC IN THE AREA (CLASS B AIRSPACE) WAS THE FALCON JET NOW WELL AHEAD OF US. I ALSO THOUGHT OF SOME TYPE OF MUFFLED EXPLOSION. WE HAD THE OXYGEN SVCED PRIOR TO THE FLT. I INSTRUCTED THE FO TO CHK THE OXYGEN QUANTITY INDICATOR -- IT WAS NORMAL. HYDS WERE ALSO NORMAL. I THOUGHT OF A POSSIBLE CATASTROPHIC TIRE FAILURE INSIDE THE WHEEL WELL; BUT THAT WOULD HAVE CERTAINLY CAUSED VISIBLE DAMAGE AS WELL AS POSSIBLE ENG PROBS. I FINALLY THOUGHT OF A LARGE BIRD STRIKE (IE; GOOSE); ALTHOUGH THIS SEEMED UNLIKELY AT THE ALT WE WERE AT; AND ALSO THE FACT THAT WE WERE NIGHT IMC AT THE TIME. I LEVELED OFF AT 9000 FT AND ENGAGED THE AUTOPLT. I INSTRUCTED THE FLT ATTENDANT TO PREPARE THE CABIN FOR LNDG AND TO INFORM THE PAX THAT WE WERE RETURNING TO LAND. I THEN TOLD THE FO TO INFORM ATC THAT WE WANTED TO RETURN TO DTW. AS A PRECAUTION; I ASKED DTW TO ROLL THE TRUCKS. I THEN TRANSFERRED CTL OF THE ACFT TO THE FO. I CALLED COMPANY DISPATCH AND MAINT CTL AND INFORMED THEM OF THE SIT. THE APCH AND LNDG WAS UNEVENTFUL. WE THEN TAXIED INTO THE GATE WHERE MAINT WAS WAITING FOR US. A DETAILED INSPECTION OF THE AIRPLANE REVEALED NO DAMAGE OR ABNORMALITIES. I REQUESTED THAT SVC PANELS TO THE BATTERY COMPARTMENTS BE REMOVED AND INSPECTED. STILL NOTHING. ALL CARGO HOLDS WERE INSPECTED AS WELL AS CTL CABLES. MAINT WAS SATISFIED THAT THE ACFT WAS AIRWORTHY AND SIGNED THE ACFT OFF. I ACCEPTED THE ACFT AND COMPLETED THE FLT TO SCE UNEVENTFULLY. A PAX ON BOARD STATED TO ME THAT; IN HIS 40 YRS OF FLYING AIRPLANES; HE NEVER HEARD ANYTHING LIKE THAT. AS FOR MYSELF AND MY CREW; WE REMAINED PUZZLED AS TO WHAT REALLY TRANSPIRED THAT EVENING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR DOES NOT KNOW WHAT CAUSED THE 'THING THAT WENT BUMP IN THE NIGHT.' HE AGREES THAT IT MIGHT HAVE BEEN CARGO SHIFTING IN THE BELLY. ALL CARGO ON HIS SAAB 340S IF FREE LOADED. THERE WAS NO RESIDUE OF FEATHERS ON THE BELLY OF THE ACFT. SEVERAL PAX MENTIONED THAT THEY HAD NEVER FELT ANYTHING LIKE THIS BEFORE.