Revving Shuttle SN45G fan

Not quite sure why this is happening. But on my Shuttle the fan is Revving up and down up and down. A temperature check seems to have the temperature under load as 50/51 C.
I thought I'd found the solution to the noisy fan when disabled the Smart Fan feature in the BIOS. However this isn't so. After about 30 minutes of use you can hear the fan roaring up and down like its on a threshold.
It does this on the settings in the BIOS for the fan, Smart, Ultra Low, Low, and Medium.

How can I minimise the noise from the fan?

(Temperature right now is 51 C on Ultra Low and its doing its revving thing :( )

My SN45G does this too.
I tried SpeedFan but you only seem to get a fast speed or slow speed, even when SpeeFan apparently changes the speed in 5% increments.

I would also appreciated a solution :)

Mars

its probably to do with the useless stock sunon fan. th is was one of the first things i swapped out in my SN45

If you check the settings, I think you'll find that even without SpeedFan there is some fan speed change with temperature, as over a certain temperature the fan goes to full speed. Sorry, can't check what that temperature is, cos my SN45G just died (typical isn't it :p )

connect the fan directly to the psu either 12v 7v or 5v and you will aviod the fan changing speeds.

As mentioned in bios there should be a adjustable setting to trigger at what
temp the fan kicks to full power.