Get a VW Passat TDI. I am getting 50mpg for around 80% highway driving with mine, using mild hypermiling (no P&G) and it is nice and quiet. Not buzzy at all either; it's geared quite high. Plenty of torque when you need it too. You don't need a 6 cylinder engine for quiet. Should be able to get a good deal on one these days...
Mine is a manual transmission, though. These do somewhat better than the automatics.