@TK42 

Turn off 2.4GHz on Main Network and if Guest Network enabled - on Guest Network, too. See if it helps. WiFi 5GHz produces much less interference.

  @Alexandre. 
I have tried that and it makes no difference.  
 

???

  @TK42 

It is also possible that powerline link between Deco units creates interference. 

Run this test: power off all Satellite Deco. Leave just Main Deco. If noise on your radio system disappears - that would mean powerline link between Main and Satellite Decos is what triggers that issue. It would also mean Deco PX50 and your radio system are incompatible.

  @Alexandre. 

Thanks for that.  I have tried to allow for that powerlink conflict by connecting each of the satellite Decos to an ethernet cabled connection.  As I underfstand it, if a cable connection is in place, the Powerline backhaul stops operating.  Unfortunately, that had no impact. 

I have now moved the master xdeco away from the 'cupbiard' where the router and the radio setup is positioned.  That works.  The downside is that the distance from the master to the first satellite deco is now such that it is getting a weaker signal.  With the Master being away from the cupboard, I cannot connect the sattelite's by cable so cannot take advantage of that route. 

ANy other suggestions would be gratefully received, like is there a way to remotely connect the master to the satellites using ethernet cables through a switch??  i.e. I can connect the main incoming router to a switch, then connect the master Deco and the satellites to that same switch but that does not seem to work. 

  @TK42 

Configure Deco mesh to run in Access Poin mode, and you can wire them with Ethernet cable any way you want. As long as switch is compatible with Deco Ethernet backhaul.

More on the topic of wiring Deco units can be found here: General questions about Ethernet Backhaul feature on your Deco

  @TK42 I think it's just the nature of the beast by sending data down mains cables since they are not twisted together, it's physics.