I'm just going to copy paste what I said in an earlier thread about fixing busted fans. It may breathe new life into your fans, or they may be done and need replacement.
Not necessary to replace the fan. I just bought a 5970 for $200 shipped that had a bad fan making loud noises too. Do what I do and take the sticker off the face of the fan and drill a TINY hole about 3-4 mm offcenter. Stop immediately when you feel the drill fall through. Put in about 5-7 drops of 3-in-1 multipurpose oil, wipe up the excess really good, replace sticker (or put a small piece of packing/invisible tape on the hole if there is no sticker or the sticker is ruined). Hand spin it a couple dozen times. Bam, you have a fan that performs as if it were new.
This works for reference blower type fans, or the regular fans on non-reference cards (or any other DC computer fan really). Done this to about 6 different fans so far, all work like a charm.