I think C903 may be onto something.
I know with my patriot I can hear (my wife confirmed this w/out me giving a leading question) the difference between a 420 grain and 426 grain arrow. Granted the 426 is a tad longer all else is equal. The bow just shoots it quieter. When I step down to a 395 grain arrow it is louder, but not that much, just somewhat louder.

If your shooting anywhere near IBO, try adding some weight (heavier tip is easiest, vanes over feathers is next) or just grab a heavier arrow and see what it does.

Aside from that- what everone else said.