Hello. I've got a noise I'm trying to diagnose. Thought I'd ask about it here before heading to a mechanic. When I hit about 35 mph I start to hear a hum sound that we've never heard before (my wife says she can here it at slower speeds, but I can't). The sound gets higher in pitch as the car speed increases. The RPMs don't affect it (i.e. when the car shifts gears, no change in the sound). If I shift into neutral and take my foot off the gas, the sound doesn't change. Seems to be tied entirely to the speed of the car, not related to RPMs, throttle, etc. Any thoughts?