A ground loop isolator is the answer. You can buy one for under $10.00 at any electronic parts store. Just attach it to the coax coming into your home and the cable going to your box. Most power conditioners already have them built in and they give you added protection, but at a price.

I own a Monster Power HTS 3600 MKII and it has worked great for me, but it also costs about $300.00-$400.00. It will clean up your power and eliminate that hum as well improve your video and audio performance. The video will be the biggest improvement, but the isolator is the budget fix that will work.