I've got a hum in my Sunfire sub.....

Those suggesting a ground loop are probably correct IF the hum is there when the sub is plugged in to another sub out connection as well. I interpreted your description as saying it was ONLY present when plugged in to the front sub out. Most HT recievers have a rear sub out as well. If my reading was incorrect, then I vote with the ground loop group ;~)

Mine doesn't know the words either! Actually a buddy of mine has one and it was doing the same thing. He contacted Sunfire they have a flat fee of 250 or 295 I forget, but they will repair it and in his case they put in new updated amplifier. He say's it performs better than it ever did.

Theo is on the right track if you have the money for the upgrade. Otherwise, Sunfire will only charge you shipping to recap/repair/check-out the unit. Many of the Sunfire's have hum problems and the company is great about helping to cure it. I sent mine back, they did a lot of work and only charged me shipping!

...it turned out to be my amp for HD OTA signal. As soon as I unplugged it, it stopped. I'm not sure what to do now though. I love my HD.

Ground loop isolater in the coax line to/from your HD signal.