The rattle could be caused by a number of things:
- the fan has worn from use and has a bit of play in it. This is non-adjustable. Allowing it to continue rattling at that speed may cause further wear.
- the shroud that covers the fan(s) may be slightly loose. Make sure all screws are snug.
- the fan is mounted loosely. You should be able to snug that down.
- a blade(s) on the fan may be damaged or bent. Make sure everything looks symetrical.
If you can't seem to correct it with any of those possibilities, you could just set a fan curve for the gpu that avoids that un-sweet spot..say goes to 70-75% up to a certain temp, then goes to 90% for any temperature higher than that