I have a Lexus ls 400 with 138,000 miles. It has run perfectly for many years. Gradually I have noticed a vibration as I accelerate. The vibration is slight and comes and goes as I acclerate through the rev band. It finally disappears past 70 mph. Acclerating between 60 and 70 the vibration is most noticable. It isn't a wheel balance problem, but feels more like a drivetrain problem. Any ideas? thanks