Are you shooting with a Zebra string on the FX? Personally, I think that the Zebra strings are noisier than other brands. Don't have any data to back this up, just a little bit of experience. After my hunting partner replaced his original Hoyt factory string with a Zebra, he thought that his bow blew up the first time he shot with the new string. He noticed a lot more string vibration.

I have a Zebra on my MQ-32 and I think it is noisy as heck without any string silencers on it. I put a cat's whisker or two on it and it quiets right down. I don't use the rubber tubing to align my sight. I hate that stuff so I use a TRU Speed Nok that aligns my sight.

p.s.: I think that something is not quite right if you are dropping 5 inches from 20 to 25 yards. Could you be tail high? Just a thought. Good luck.