Type Cii has problems. Yeah, IT DOES.

[~Phoenix Death~]

During a sexy R U R' U', halfway through R', the corner starts being able to rattle.
Should I tighten or loosen the screws, cause the cube is stiff too.

 

[4Chan]

Replace spring with original type C spring.

 

[Faz]

Hmm, well if I try to cut corners on my cubes like that, they are able to rattle too.

 

[eastamazonantidote]

It sounds like you have witnessed why FII's and CII's are overrated: they pop and rattle and begin to lock more and more until it becomes a pain in the rear to finish a solve. My FII has more issues with this, though the CII is beginning to show signs of wear only 2 weeks in.

I would change the core or at least the springs. Tighten the tension slightly or even loosen it a bit.

 

[~Phoenix Death~]

Fixed it.
I now why now. Cause I sanded it a bit wee much, giving it more spac and air to move around.