When driving at highway speeds, usually only after it is up to operating temperature, I sometimes hear a low rumbling like noise coming from the front of the car. It generally happens in the range of 77-85+ mph. I feel a slight vibration in the gas pedal; nothing really in the steering. It's not due to road surface and doesn't seem like an alignment issue since I don't feel anything in the steering. Slowing down to 60-65 it goes away. The car feels like there also might be a bit of power loss when this happens; but I could be imagining that aspect of it. The low rumble is noticeable even with the radio at a normal volume. I had the pulsing noise with the AC off and got that fixed by the dealer replacing my AC compressor; I thought perhaps that would fix the rumble, but it did not. Has anyone else experienced something like this? If so, what was it? It is intermittent so I expect the dealer will have trouble reproducing.