I know this hasn't been posted to for a while, but I actually found a few tricks that I wanted to share here for anyone that comes across this post...  (because they are tired of their S9's wailing at them possibly, like me)

I have 2 S9's from the Jan batch, and both have a distinct whistle in them..  One has two Nidec fans, the other has a Nidec and a Yate Loon..  They both whistle horribly at full spin and maybe within 55% of full spin all the way to the top (full spin in my head being 6000rpm)...

This at least worked for ME...

The S9 with the two different types of fans...  It came with a Nidec on front and a Yate Loon on back..  I just swapped them..   Putting the Yate on the front and the Nidec on the back still gave it a whistle at full spin, but running between 4000rpm and 5000rpm where they stay in my setup, no more whistle... 

The S9 with two Nidec fans...  I repositioned the center board just a little bit, by wedging something VERY small (I used a rolled up dot of gapher tape) under the card right at the edge...  This made the center card sit VERY slightly higher in the front of the ASIC than in the rear, and changed the path of the airflow very slightly..  There isn't much play so it doesn't change it much, but that simple change also killed the whistle in mine at the mining spin speed (this one goes between 3800rpm and 4800rpm typically)...

More or less, both "fixes" just changed the wind going through the ASIC just a little bit (which was enough in my case)...   

Can't say this will help anyone here, but those small changes made a huge difference for my noise level and sanity...  Maybe these ideas will help a few others come up with other ideas along these lines (if these don't help of course)... 

Happy mining everyone....  and may it be a more silent mining for you all..