After adding two more link rooms with TV's I have hum in the system :-(

So my first guess was this could be related to ground loops from aerial connection on the one hand and ML connection on the other. As ML is a differential (is this the correct word?) bus I was wondering what the ground connection is for?

Do you also experience such problems and how did you solve them? Could it be a solution to cut the ML ground connection and use the aerial ground instead? A ground isolator in the aerial is not possible as I have a satellite system and need the DC connection for signalling purposes.

Many thanks,

Marcus

Think you are missing a Yellow or RED wire in you masterlink cable. If it's in all your link rooms it's properly to your radio or there is a short cut some where.

(With RED wire I mean the RED or the W/RED)

see if it's L or R speaker there is noise in? 

It's in both speakers and not that heavy if one of the left or right wires are broken (then the level would also be lower). It is there but not that dominant, if playing at normal volumes it is in the background, but becomes present at low volume parts of the music or during the pauses.

Thought it could be related the ground as there are different grounds connected to most of the rooms (aerial and ML), which could (in theory should) result in hum due to ground loops......

You can try and unplug products form Master link and see if it's stops. or if a red and a yellow wire is shortcut... is it cat6 or 7 you use instead of master link cable? since i ask if it both on TV and radio is it cut be internal in the radio the problem lies..