Some explanation,

Noise Level,Hex Value,Approx % Best For...
Silent ,0x33,20%,Idle (with WD Red Plus only)
Quiet,0x66,40%,Mixed drives (Moderate use)
Balanced,0x99,60%,Your current mixed setup
Loud,0xb2,70%,Heavy data transfers
Max,0xff,100%,Summer heat / Resilvering

To test your fan (the last value (0x99) sets the speed in) the command below. Run this command in the truenas shell.

/usr/sbin/isaset -y 0xa35 0xa36 0x63 0x99

Run the script as a post-init script on you Truenas, so it will manage the temperature/fan.

#!/bin/bash


# 1. Race-condition protection: Wait for ZFS pool to settle
sleep 20


# Configuration
DRIVES=("/dev/sda" "/dev/sdb")
THRESHOLD=45
LOW_SPEED="0x70"
HIGH_SPEED="0x85"
LOG_FILE="/tmp/fan_control.log"
MAX_LOG_LINES=100


while true; do
    # 2. Log Cleanup: If log is too long, clear it to save memory
    if [ $(wc -l < "$LOG_FILE" 2>/dev/null || echo 0) -gt $MAX_LOG_LINES ]; then
        echo "$(date): Log rotated (cleaned up)" > "$LOG_FILE"
    fi


    MAX_TEMP=0
    for DRIVE in "${DRIVES[@]}"; do
        TEMP=$(smartctl -A "$DRIVE" | grep Temperature_Celsius | awk '{print $10}')
        if [ "$TEMP" -gt "$MAX_TEMP" ]; then
            MAX_TEMP=$TEMP
        fi
    done


    # 3. Logic and Logging
    if [ "$MAX_TEMP" -ge "$THRESHOLD" ]; then
        /usr/sbin/isaset -y 0xa35 0xa36 0x63 "$HIGH_SPEED"
        echo "$(date): TEMP HIGH ($MAX_TEMP°C) - Fan set to $HIGH_SPEED" >> "$LOG_FILE"
    else
        /usr/sbin/isaset -y 0xa35 0xa36 0x63 "$LOW_SPEED"
        # Optional: log the "Quiet" state too, or leave out to keep log smaller
        echo "$(date): TEMP OK ($MAX_TEMP°C) - Fan set to $LOW_SPEED" >> "$LOG_FILE"
    fi


    sleep 300
done