I managed to underclock my autotune 13TH S9 by first upgrading the firmware to the 550MHz fixed frequency firmware.
Thereafter ssh into the miner. edit /config/bmminer.conf and change
"bitmain-freq" : "475",
Also one can change the directions of both the fans and the miner should reduce fan speed.
Do not change the directions before underclocking though, as the miner will overheat.
I tried that on mine and both would go back to auto tuning the frequencies.