The RF 600mm F/4 L IS USM is supper quit when the IS is on.
Bryan's review mentioned that he heard a click and whirling noise in his review.

I called Canon Service to ask, they said that it is supposed to be extremely quite and what I am not hearing is normal. I couldn't get much out of them other than this. I was hoping that they would give an explanation of how it works.

Looking at the spec sheet for the EOS R5 it appears that the IBIS is handling quit a bit of the IS.
All RF lenses with IS says that Pitch/Yaw is Coordinated Control, Optical IS + in body IS.

It made me wonder if the RF 600mm F/4 has a different type of stabilization than the EF system.
One that is designed to work with IBIS system rather than a stand-alone.

I have tried to find info online about the RF 600mm F/4 and its system, not much out there yet.

Other lenses are discussed in the R5 Spec Sheet:

It seems the workload for stabilization is divided up between the In Body and In Lens.
If anyone knows of a good article that describes what "coordinated control" is and how they two are working together it might be an interesting read.