Rats brux, (chat their teeth together,) to keep their teeth down, but also when they are happy. If they are hiding and chattering, they are probably just enjoying themselves. Also, two rats will often squeak when wrestling with other rats, as well as during cleaning sessions to let the other rat know if they are too rough with cleaning or play.
Nugget often cleans Silvia a little too much ans Silvia complains to her to let her know. If it's during cleaning, don't worry, because if it was hurting the other rat enough to bother them, they would wrestle with the rat to make it stop, or run way to tell them they've had enough, but Silvia often lays with her eyes closed, squeaking every now and then while Nugget cleans her with out even caring enough to open her eyes. While playing, I don't usually worry about squeaking unless it's really frequent, because they play fast, so wrestling always looks aggressive because of their energy level.
They get carried away, but unless you see one of the rats bleeding or one of the rats with the other rat's fur in it's mouth or claws, don't be worried. Usually if you flash a light at them, (my rats play a lot when I'm trying to sleep...,) or bang the cage, and they both stop and look over at you confused, then they are just playing, and may even go back to playing until you try to get their attention again. If the fighting persists and looks like one rat is injuring the other, and the squeaking is constant, then maybe be worried.
NEVER try to stop them by grabbing or touching them, as you can get bit in the cross fire rather seriously. Try to separate them with a piece of cardboard or a toy, (NEVER your hand,) and remove the rat who is the rat getting beaten up, keeping them with you, or if too scared, in an other cage or box of blankets. DO NOT try to pick up or touch the aggressive rat, even if they are sweet and calm normally, because they are full of adrenalin and need to calm down.
If both rats are aggressive, or if you are not sure who is the one losing the fight, separate them as described earlier, if you can, then take one out, (if injured, take the non injured one, though instinctively you want to help the hurt one, because a hurt rat is more likely to attack you when cornered after a fight, even if they are sweet normally,) using a folded towel or shirt, keeping them away from your hand as much as possible.
Once separated, give them time to calm down. If one of them is injured, wait until they are calm before trying to treat them. If they are seriously, and possibly fatally, wounded, then try to get it to a vet immediately. If rats fight bad enough to injure each other, then do not attempt to keep them in the same cage any more. They have showed they are endangering each other, and possibly need to be housed with out an other rat, and there is no point in going through this problem again.