I did soo many PDI on RC5xxx. They're noisy from the factory. I don't know why.
Maybe thicker oil would do the trick. Customers don't enjoy the fact that they're noisy like that. Many service calls for noisy drive unit and I have to tell customers that its normal. Wish they were more like the Raymond 4250...no noise at all.

Very common on the RC55xx to have what seems like excessive gear noise from both drive units, thats just how they are straight from the factory. Difficult to say if you have an actual issue or a standard RC....