The issue of a CPU fan running fast and slow with loud noise can be quite bothersome and may indicate several problems.

One possible reason could be dust accumulation. Over time, dust can build up on the fan blades and in the heat sink, affecting the fan's balance and airflow. This can cause the fan to struggle and vary its speed, resulting in the loud noise. To solve this, you need to clean the fan and heat sink. Use a compressed air can to blow out the dust or carefully disassemble the fan if possible and clean it with a soft brush. Another reason could be a problem with the fan motor or its bearings. If the motor is wearing out or the bearings are damaged, it can lead to irregular speed and noise. In this case, it may be necessary to replace the fan. Additionally, incorrect BIOS settings or a malfunctioning temperature sensor can also cause the fan to behave erratically. Check the BIOS settings to ensure the fan speed control is configured correctly. If the problem persists after trying these steps, it may be advisable to seek professional help or consult the manufacturer's support. Overall, addressing the issue of a noisy and speed-varying CPU fan is important to maintain a quiet and stable computing environment and to prevent potential damage to the CPU due to improper cooling.