During a recent 1,000 mile trip, I noticed my Model S 60D would make a faint noise under acceleration from the front of the car that sounded a little like a diesel engine. Often times, it would be masked by tire or wind noise. The more I pushed the accelerator, the louder the noise would become, but it continued at the same pitch and speed. It only seemed to manifest when the car had been going 70+ mph for a while and disappeared once the car was going slower for a while. It didn't really sound like the milling noise people have posted on YouTube. Has anyone experienced this?