Hi, thanks for the replies.
Yes, it is plugged into a four pin header which is the only CPU fan header on the board.
CPU Q-Fan was always enabled and setting it to silent doesn't make a difference.
I don't understand your third point, I couldn't find anything like that in BIOS. Could you explain?
It only speeds up to 1500 RPM when Windows starts, it sits at 500 in BIOS and while Windows is starting. So do you think it is something within Windows that is causing it?