I have a 70# FC400 also. My arrows are at 383 gr. total weight. A little bit louder than what I could accomplish with a heavier arrow, but it's a good compromise between silence and speed. It keeps the bow plenty quiet for hunting purposes. On initial set-up, my bow was fairly loud to me. It took a bit of cable twisting to get the cams timed, but once they were, it was significantly quieter. I think this might be part of your problem. I have limbsavers, cat whiskers, and a stabilizer for noise and vibration damping. Once the cams were set right, that's all it took to make it pretty darn quiet. Also, getting the cams timed just right should give you a speed increase. Increasing speed and decreasing noise at the same time is a great thing! lol
2006 Rytera Bullet X 60#---2008 Moab 70#
2008/12 Firecat 60#--- 2010 Warthog 70#
2010/2012 Alien X 65# 2011 Onzas 60 & 70#
2009 Warthog 70#---2009 Firehawk 70#
2010 Strother SR-71 65# & 2012 Strother Rush 65#
I hunt, therefore I eat. lol