I had a very similar thing happen to me. I have a PVR 250 and recently moved into a different house. A lot of the outlets only had 2 prongs, so I have been slowly replacing them with 3 prong. There is a ground wire coming to each box, so I was able to add a ground to the new outlets I put in.
I have 2 Bell ExpressVu receivers and found that when I had them both operational, I would get this annoying hum in my recordings, so I had to disconnect one when I wanted to record anything. I eventually got around to replacing the outlet that my computer was plugged into and now there is no more hum.
The other night though, the hum returned, but has since gone away so I am hoping that doesn't keep happening. I pulled my hair out though trying to figure out what was causing it...trying all sorts of combinations of cables, checking all the connections to see if anything was loose, checking the speaker wire, etc. So when I hear people talk about the ground, that does make sense as that appears what it was for me.