Is the noise from those metal halides coming in on the mic lines (Mic's on stage--->mixer) or on the lines that go from the mixer to the amps? This will split your troubleshooting area in half. Try this: Mute the sound board, making sure all your outputs are turned all the way off, auxes, monitors, everything. Turn on all the amps, and then start up those MH's...do you hear the interference? If yes, it's on the lines from the mixer to the amps, check the shields at both ends of these cables...one side should not be connected to anything, the other should be connected to pin 1 of the XLR connector (3 pin, mic style connector, the pins are labeled). The shield should never be grounded to conduits, boxes, etc...think of it as a neutral, you only tie it to ground (pin 1)in one spot, and it stays completely isolated therafter.
This is not the case on mic cables, they need the shield to be connected all the way through...but it still should be isolated from chassis grounds, etc. Treat it just like a signal wire.
a board like this one that has a lot of info on live audio is
http://srforums.prosoundweb.com/ check out some of the past posts, there are many others in your situation!
Oh yeah, make sure all involved equipment has its ground pin intact and that you have good grounding to all outlets. Also, something I used to do a lot is check the voltage at outlets from Neutral to ground with a digital meter, shouldn't be more than a volt or 2, if you have weird things going on, this may indicate problems with wiring on your side of things.
Let us know what you find out!
