Latitude 7440 problem hum when using HDMI port and external screen in venue.

Latitude 7440 problem hum when using HDMI port and external screen in venue.

The setup is..

We have a sound system set up on stage, mixer etc plugged into powered speakers.

We run a USB from 7440 to an external sound card to the mixer which then goes to the powered speakers.

It all runs beautifully, no hum, just great.. until !

We plug a lead from the venue fixed large screen into the 7440 HDMI port.

Then we get a low hum in the speakers, not too loud, but still there.

Take the HDMI out and it is silent!

I have tried changing power sources to get another circuit, and all other leads, but it is still the same. Noise until I take the HDMI out!

Any ideas what it may be and how to fix.

Thanks

oz