I have a Sapphire RX 580 4gb gpu with two built in fans. When I boot my PC only one of the fans spins, at full speed. The other one doesn't move. This means the gpu is very loud and distracting, any fixes?
Sounds like either the fan went bad or a connector came lose in the card. If its still under warranty send it in, if its not you could pay to get it repaired (although the cost may not be worth it) or if your comfortable open the card up and see if you can find a lose connector or take a multi-meter to the fan.
Both fans should spin up for a second at the start up, and if one fan is on full while the other isn't moving it definitely sounds like a hardware issue.
Hmm sounds like normal operation then.
You could try MSI Afterburner to set a custom fan curve or use Sapphire's Trixx software (I know nothing about it) and dial in a fan speed that works for you.