I know I've done this setup before, but
last night had big problems......

shooting with a battery powered camera,
boom mike attached via XLR adapter, and
monitoring video out to a Sony monitor,which was plugged in (AC)---

the video out cable to the monitor was creating the dreaded 'hum'...remove the cable
from the jack, hum gone

does this mean I HAVE to run my external
monitor off a battery as well?

Use an isolation device or a ground-lifter.

Mike,

Where do you find an isolation device or ground lifter? Do you use any specific one? Thanks...

http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=161090&is=REG

Of course this one is a 1/4 inch jack I/O but with adapters it will do the job. I always carry one around if I am plugging into someone elses board.

Also, search around TecNec (www.tecnec.com), They have a bunch of different Ground-loop and Hum eliminators in their catalog, and i'm sure on their website as well.