It should have gone back to Auto when the power came back on but just to be sure I unplugged it and plugged it back in again. That put it in auto and it was quiet for awhile. Then the rumble started again. I think it starts after the bass first gets directed to the sub.
I adjusted the volume. If I lower it the sound is still there but just not as loud. Raising the volume just makes it louder.
I thought maybe the power cord and cable were touching but that's not the problem. The rumble does go away completely if I turn the AVR off (but it rumbles if I mute the AVR).
It's not really a background noise. I tried a CD that has a lot of bass in it. When the bass is booming you can't hear the rumble at all (even up close). When the bass isn't going to the sub you can hear the rumble. That's what makes it annoying. When I first heard the noise I thought the A/C was running.