Fan problem

tm_simpson

Dell Dimension 5150

Straight away when I turn on the computer the fan in front of the heat sink for the cpu is on full power and very noisy. I never used to be able to hear it at all

I have followed the noisy fan troubleshooting steps on dell support website.
- removed and blown all the dust out of my case. There wasn't much as I have only had the computer a little over a year and always did this regularly.
- I have run the dell utility tool by pressing F12 and there are no errors on the fan test.
When it tests the fan on low speed the sound is normal. However for some reason my system thinks it needs the fan to be on full. I don’t know why?

Any help much appreciated

Jzpop

As I am sure you are aware, the fan spins to pull cool air over the CPU, and keep it within the design operating temperature.

If the fan is spinning fast all of the time, there has to be a fault. Personally, I would be asking Dell to send an engineer to investigate. If you want / need to check yourself, I would suggest the most likely causes include...

Faulty motherboard sensor (sending too-high readings ref the CPU temp)
Poorly seated heatsink
Blocked ducting (within the duct, or at the inlet or outlet vents)
Very high ambient temperature
Very high CPU load (check running processes / CPU load)

Can you remove the PC casing and duct? If so, does the fan still spin at full speed, with low CPU load? Can you point an external room-cooling fan to pass air over the heatsink, and see whether that lowers the fan speed?