Highly recommend 6" AC Infinity Cloudline blower fan - runs very quiet compared to stock fans. Will need a shroud to interface between blower and miner. You can get those from nostr:npub1lrjgpa9m2dl6esdah5pq7m005ymwh4glq4aet76q2zlz2f83esfsa2zvrj , eBay, etc. Will also need spoofers to trick S9 into thinking fans are still connected. You can also get those and other fun goodies nostr:npub1lrjgpa9m2dl6esdah5pq7m005ymwh4glq4aet76q2zlz2f83esfsa2zvrj and other places.
Discussion
That solution is probably the best in terms of noise reduction. If you go to cryptocloaks site they have a guide for upgrading to 140mm fans. Which is prob cheaper/easier.
You'll need two 120mm to 140mm adapters (can be printed or bought) as well as two 140mm fans and dont forget to upgrade the 60mm power supply fan too. I suggest the Noctua NF-A14 industrial/heavyduty 3k rpm version (3K rpm is important so the fan can spin fast enough to dissipate heat at max asic power load. Careful cause they also sell 2k rpm and standard fans). I used the Noctua NF-A6x25 for the psu.
If you upgrade the psu fan you'll either need to solder or use an adapter.
Links below
https://www.amazon.com/gp/product/B00KFCRF1A/