The ND5XS does have a ground switch on the back with options for “floating” and “chassis”. And the device must be plugged in to an earthed 3 pole mains socket. It should be set to “chassis” and the supplied 5 pin DIN that came in the box has a dedicated single signal ground pin. As such I agreed with @DiggyGun that as a starting point, the supplied DIN cable should be used - at least until the problem is root caused.
If the problem is only at the buyers home, and you never experienced it at your home, there could be a problem in the power system in the buyers home and a qualified electrician should check it out. Maybe the mains power system in the buyers house isn’t earthed properly or some other issue.
As I said I’m helping to setup my former 5si which I had to sell. The source is Yamaha-303, it has 2 pin mains.
The ground cable is connected somehow to yamaha, using the cable like this Massive sudden hum - frightening! - #15 by james_n
It is a bit difficult to make photos, by some reasons, I’m just helping the buyer as I like this amp very much.
Yamaha uses two pin socket and the ground is made the same way as here Massive sudden hum - frightening! - #15 by james_n
Actually, it was working for me, when I was using another Yamaha. But now it works strange when setup in another home.
No difference, if this ground cable is connected either to yamaha or to 5si - the hum disappears and then slowly returns and grows.
Does the same happen with no source attached to the Nait?
You say it’s static on the volume knob, which is plastic so might be able to hold static but that doesn’t explain where it’s coming from. Have you access to a multimeter to check it’s not AC or a charge on the case of the Nait?
Is the static rather on you and discharging through the Nait? When I used to listen from a chair with a nylon cover in dry weather I’d get a belt when I touched the amplifier ps, which I did before handling the tone arm or there was a loud crack through the speakers.
Oh right. Gotcha. Hmmm, I’ve certainly connected non earthed stuff including Yamaha sources to Naim before. As yet not had any problem with hum.
Another thread recently had a member with a similar problem and it was a faulty RCA cable. They used a different RCA lead and the problem went away. Can you test with another cable? Even those cheap free ones would help for troubleshooting.
From my experience, all yamahas(wxad-10, wxc-50, 303) and some other devices(elac discovery, arcam cds 50, node, svs 1000 sub, etc) gave humming being connected to 5si and every time I solved this by adding the additional ground cable as described in the post by the link above. This issue is described lots of time right on this forum.
This time, at the buyer’s home, is a bit different behavior - the hum goes out when the earthing cable attached, and then slowly returns and gradually increases for about half an hour.
