I thought the sub woofer was going as I was getting a rumbling noise. It isn't very loud, but it is such a low tone I can feel it slightly in the steering wheel.
So, I eventually turned off the radio, but it didn't stop. It's a low rumbling noise that comes and goes for no apparent reason. It is not speed related. If I go at a steady speed (cruise control) and the noise starts, slightly pressing the gas pedal for a second makes it stop for a short time, but then it comes back.