A B737-700'S ENGS ACCUMULATED FAN BLADE ICE DURING TAXI AND EXTENDED HOLD FOR RVR IMPROVEMENT IN DENSE FOG AND FREEZING TEMPS.

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PRIOR TO PUSHBACK PREFLT INDICATED NEED FOR DECIDING DUE FROST. TEMP WAS -1 DEG C DEW POINT -1 DEG C. VISIBILITY WAS INITIALLY RPTED AS 1/4 MI. AFTER DEICING AND DURING TAXI TO RWY 3; VISIBILITY DROPPED TO 600-800 FT RVR. TWR CLRED US INTO POS AND HOLD ON RWY 3 AND SAID 'RVR 600 FT.' FURTHER INQUIRY REVEALED MID RVR TO BE 500 FT. NOW CLRED INTO POS AND HOLD WHILE WAITING FOR RVR TO IMPROVE. HOLDOVER TIME OF APPROX 17 MINS WAS NOW EXCEEDED. I (FO) WENT BACK TO PERFORM CONTAMINATION CHK. NO CONTAMINATION. SHORTLY AFTER I WAS AGAIN SEATED; WE BOTH NOTICED A SLIGHT VIBRATION. ALL ENG PARAMETERS APPEARED NORMAL. CAPT STARTED TO RUN UP ENGS FOR CLRING PROC AND THOUGHT BETTER OF IT AT APPROX 40% N1. CAPT AND I DISCUSSED IT FURTHER AND DECIDED TO RETURN TO GATE SUSPECTING FAN BLADE ICING. AFTER RETURNING TO GATE INSPECTION SHOWED 1/4 TO 3/8 INCH OF ICE ON BOTH FAN AND STATOR AS WELL AS CONSIDERABLE BUILD UP OF ICE ON THE BACK OF FAN BLADES. CONTACTED MAINT CTL AND DISPATCH. MECHS HEATED THE ENGS WITH CONDITIONED AIR UNITS AND AFTER ANOTHER DEICING OF THE AIRFRAME WE DEPARTED WITH NO FURTHER PROBS.