I just got back form a trip to Bar Harbor ME on my 2011 RT-S, total about 600 miles and on the way back I noticed a vibration that comes and goes. It feels like running over the rumble strips they grind into the roadway to wake you up as you approach toll booths. I get it around 75-80 mph, on glass smooth roads even, but if I accelerate or decel it goes away until I settle on a steady speed again. I suspect it is a belt vibe and perhaps the 600 mile service will address it. Is this common?