Hello Kelvin Wong! Before you try anything drastic, I would recommend that you just give your Mac Pro a good dusting. This is usually the problem in these types of machines and will usually drop your temperatures about ten degrees. While you are in your Mac Pro, I would also recommend replacing the thermal paste on at least on the CPU. The GPU could also be done as well if it is running hot. Chances are that the thermal paste in your machine is dried up. After sealing the Mac Pro back up and powering it on, I would recommend installing a free application called Macs Fan Control. It is one of the best fan control utilities for Macs and it works really well.
You can find the download link for Macs Fan Control here: https://crystalidea.com/macs-fan-control...
Check the CPU die or core temperatures as well as the temperature of other temperature probes inside of your Mac Pro. These are listed in the right panel of the application.
It is considered normal for a Mac to idle at 40-50 degrees Celsius depending on what type you have. I would say 50 degrees is too hot for a desktop Mac to idle at, so you can change the fan settings to make it idle cooler. You will need to do a little bit of experimentation with this, as I have never owned a “trashcan” style Mac in my life. What I can tell you is that the CPU core or die temperatures should never go over 80 degrees Celsius under load, especially in a desktop. I would recommend changing the fan curve for the exhaust fan in your Mac Pro. To do this, click on the custom control button for your Mac Pro’s exhaust fan. Set the control option to sensor-based value. Change the temperature sensor to either the CPU die or the CPU core average. I would recommend setting the temperature to start ramping up the fan to about 50-60 degrees Celsius depending on if you want your Mac Pro to idle cooler or quieter. Set the maximum temperature to about 80-90 degrees Celsius respectively.
Let me know if this resolves your issue. I would recommend trying this first before going and replacing the fan or heatsink. Chances are that if the fan is spinning at full speed most of the time, something is blocking its airflow or the heatsink’s heat transfer ability. Have a great rest of your week!