B737-500 CREW HAD ENG FAN BLADES THAT HAD ICE ACCUMULATION; AND WERE DAMAGED.

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AT DESTINATION R ENG WAS FOUND TO HAVE SEVERAL FAN BLADES WITH BENT TIPS; APPARENTLY FROM ICE SHED FROM FAN BLADES. HAVING BROUGHT IN A DIFFERENT AIRPLANE SHORTLY BEFORE; WE WERE AWARE THAT OUR ACFT HAD PICKED UP RIME/MIXED ICE LNDG IN FZFG. IN ZZZ; GND CREW INFORMED ME THAT THE GEAR AND ENGS HAD BEEN DEICED BEFORE I BEGAN THE WALK AROUND. DEICING FLUID WAS STILL DRIPPING FROM ENG COWLS AND THE GEAR. RADOME AND UNHEATED TAIL SURFACES WERE COATED WITH ICE. ON TAXI OUT WE WERE DEICED WITH TYPE I FLUID AT REMOTE DEICING PAD. BOTH ENGS WERE RUNNING DURING ENTIRE TAXI OUT AND DEICING. VIS WAS GOOD AND ALL PRECIP HAD CEASED FOR OUR TAXI OUT AND DEP. ENG RUN UP BEFORE START OF TKOF ROLL WAS NORMAL. AT LIFT OFF NOTICED RUMBLE THAT SOUNDED LIKE DC-10 ENG. CLBING THROUGH ABOUT 8000 FT NOTICED R ENG VIBRATION INDICATION WAS AT ABOUT 3 UNITS. THEN ENG RUMBLE CEASED AND VIBRATION LEVEL REDUCED TO LESS THAN ONE. ENG WAS NORMAL AFTER THAT. ENG ANTI-ICE WAS ON ALL DURING TAXI OUT AND CLB. FOUND OUT LATER ICE BUILDS UP BEHIND BLADES. DIDN'T LOOK REALLY CLOSE AT ENGS DURING PREFLT BECAUSE THEY ALREADY HAD BEEN DEICED. ALSO THOUGHT ENG ICE PROB WOULD BE MORE NOTICEABLE WHEN ENG IS RUN UP FOR TKOF. ALSO DID NOT KNOW ICE SHEDDING COULD DAMAGE FAN BLADES AS EASILY AS IT DID. SUPPLEMENTAL INFO FROM ACN 565520: WE ARRIVED AT THE GATE TO FIND THE PLANE COMPLETELY ICED OVER. THE GEAR AND ENGS WERE DE-ICED AT THE GATE. WE DE-ICED AT SPOT FOLLOWING THE REMOTE PAD DEICING CHKLIST. NO ABNORMALITIES WERE NOTED WHILE FOLLOWING PROCS FOR ADVERSE WX OP. WE TOOK OFF ON RWY XP WITH NO DELAY; NO VIBRATION WAS NOTED DURING A STATIC RUNUP BEFORE BRAKE RELEASE. PASSING 9500 FT IN THE CLEAR; WE TURNED OFF THE ENG ANTI-ICE AND SOON NOTICED AN UNUSUAL SOUND/RUMBLE. ALL ENG INSTRUMENTS WERE NORMAL; AND WE SUSPECTED A PACK OR DUCT PROB. THE RUMBLE WAS MOST EVIDENT IN THE BACK OF THE PLANE. A B757 FO IN THE BACK ASSISTED IN TROUBLESHOOTING AND SAID HE THOUGHT THE PROB WAS A PACK. WE CALLED DISPATCH AND ACR MAINT CTLR TRIED EVERYTHING WE COULD THINK OF; BUT NO ONE WAS SURE WHAT THE SOUND WAS. EVERYONE AGREED IT IS SAFE TO CONTINUE TO ZZZ; THE RUMBLE WAS MUCH QUIETER AT CRUISE AND VERY QUIET ON DSCNT. WE DISCOVERED BENT FAN BLADES AT ZZZ; ONE OF MANY B737'S WITH THE SAME PROB. I SHARED ALL OF THIS INFO WITH ACR FLT SAFETY AND UNION SAFETY DURING A CONFERENCE CALL ON NOV/WED/02. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE ENGS AND LNDG GEAR WERE DEICED ON THE GATE. THE RPTR SAID THIS MAY HAVE CONTRIBUTED TO THE BLADE DAMAGE INCURRED TO #2 ENG ONLY. THE RPTR STATED WITH BOTH CARGO COMPARTMENT DOORS OPEN AND THE FUEL TRUCK CONNECTED THIS CONGESTION AND TFC MAY HAVE LIMITED THE SPRAYING OF DEICER FLUID AROUND #2 ENG.