I have a Sata Hotplug Bay with a rather loud fan hotwired to the back of it. To keep the noise down, I use a udev rule in combination with this python script to only turn on the fan while there is one or more disks attached to the controller card that services the ports.

onSataHotplug.py

#!/usr/bin/python

"""
turn the fan to dc mode and off when the fan controller is initialized
/etc/udev/rules.d/97-hwmon.rules:
ACTION=="add", SUBSYSTEM=="hwmon", DRIVERS=="nct6775", ATTR{pwm5_mode}="0", ATTR{pwm5_enable}="1", ATTR{pwm5}="0"

declare the disk as hotpluggable and run the script
/etc/udev/rules.d/98-sata.rules:
ACTION=="add", SUBSYSTEM=="scsi_host", DRIVERS=="sata_sil24", ENV{UDISKS_SYSTEM_INTERNAL}="0"
ACTION=="add|remove", SUBSYSTEM=="block", KERNEL=="sd?", RUN+="/usr/local/bin/onSataHotplug.py"
"""

import os

#get list of scsi controllers
scsiControllers = os.listdir("/sys/class/scsi_host")

#get controllers with driver sil24
bayControllers = [c for c in scsiControllers \
                  if open(os.path.join(
                      "/sys/class/scsi_host", c, "proc_name")
                     ).read() == "sata_sil24\n"]

#get sil24 controllers that have a target scsi_device attached
isBayOccupied = [any(map(lambda x: x.startswith("target"),
                           os.listdir(os.path.join(
                               "/sys/bus/scsi/devices", c)))) \
              for c in bayControllers]

#get hwmon devices
hwmonDevices = os.listdir("/sys/class/hwmon/")
#filter out the Nuvoton fan controller
fanController = [c for c in hwmonDevices \
                 if open(os.path.join(
                     "/sys/class/hwmon/", c, "name")
                    ).read() == "nct6793\n"][0]

#set the pwm to on if there is one or more disk, otherwise off
with open(
    os.path.join(
        "/sys/class/hwmon", fanController, "pwm5"
    ), "w") as f:

    if any(isBayOccupied):
        f.write("100\n")
    else:
        f.write("0\n")

exit(0)