How does the noise scale with engine RPM; I assume is does? Do you also hear the noise with only the rear PTO engaged? My 62" mower noise (pretty sure it's mower and not tractor) definitely increases significantly above 2800 RPM, but doesn't sound anything as disturbing as your describing. Actually, I would say 3000-3100 rpm is just slightly louder than I was hoping for with 2600-2800 being very quiet. I will break out the sound meter tomorrow and measure. I'm fairly certain my 2305 is quieter under all conditions than my 54" 335, but it could be that the frequencies are different on the 2305 and less noticeable. I would be interested in knowing just how loud this noise is if you happen have a sound meter (e.g. from Radio Shack). I would think you would also have a stronger case if you can measure noises significantly louder than rated and to the point of increasing hearing damage.
Any case, I would quickly do the A-B compare with another 2305 and argue to swap something (mower deck, tractor), etc if the noises are significantly different. Even is the noise is not due to any actual damage occuring in the system, it sounds like besides being plain annoying, you will be forced to always wear hearing protection under all mowing conditions to avoid hearing damage. I always wore hearing protection on the 335, and while I probably should also for good measure on the 2305, I suspect I could get by without, especially if I dropped the RPMs a bit.
Finally, I think your odds of being remedied go down the longer you wait and more hours you put on the tractor.
Hopely this will yet turn out well for you...