I have completed a multi-room system which works absolutely fine until certain lights (halogen) are switched on, creating a hum in the background of the speakers local to that room(the others are fine).

We have double checked the speaker wires to make sure they are as far away as possible from any mains cables, transformers (every cable is at least a foot away).

From what I hear this is a fairly common problem although I have never experienced it before.
I can only presume it is a lighting transformer causing the interference as all the other zones appear fine (how is this interference caused?)

I would appreciate anyones expertise in solving this problem i.e. would shielding the transformer in a steel enclosure help?
Someone else suggested earthing the lighting transformer to a good earth such as a pipe would help.

Thanks for any help.