I have a hum I recently noticed from my bedroom SD Comcast DVR box(Pace TDC577X). I imagine the DVR has a spinning disk, but funny thing is it hums when it is OFF too. Probably totally unrelated to your issue, but since you guys were talking about humming, it reminded me of my hum.
DVRs are notorious for spinning their disks even when "off" (standby). They can't really be off, because they need to power themselves up in order to make scheduled recordings, and also to update their guide information.
