NAC-N 272 Popping on Volume or Selected Input Change

Greetings - My few months old 272DR has suddenly started popping who changing volume or selected input (including unused inputs). I did a forum search and found a thread called 'Another new 272 popping?' that didn't seem to relate to my situation although it did refer to another thread on the same topic that I was unable to find.

Our 272DR enjoys an XPSDR and drives a 250DR. Inputs are WiFi connected except for the Humax TV satellite receiver which is optical. Curiously the same popping can also happen when the viewed TV channel is changed on the Humax satellite receiver. This popping isn't a 'plop' but a higher frequency single crackle. It first happened a couple of days ago then the following day was okay and is now back again. The first day it popped, with the remote volume control continuously pressed, every time the digital volume display rolled over to the next digit e.g. 35 > 36, etc. It makes no difference using either the remote control or the iOS Naim App.

I've not had an opportunity to look into it to try and establish any repeatability pattern and probably won't be able to investigate for a couple of days but since the thread that I found was called 'Another new 272 popping?' is/was there a known issue of some sort?