I think you are probably the closest on this lloydjacobson. Experiencing it periodically and noting the cold weather correlation, I'm thinking it is a simple "Harmonic Resonance" with essentially, and unintentionally, the vehicle resonating at that rpm. When everything is warm particularly the rubber parts between metal parts being all soft and squishy relatively speaking, it is dampened. The colder you get, the less squishy (technical term) and the more the vibrations is transmitted to where you can feel it. And due to the nature of a vehicle and rpm moving up and down intrinsically transient unless it is right at the sweet spot for highway driving and this one is close and could be vehicle specific as in mine might not do it, but yours might as the "exact" rpm for each vehicle would not be the same. Try to chase that down as an engineer. Increase highway rpm even just a bit and it might go away along with some mpg. Even changing that harmonic thing might work for only a bit as systems tend to move toward the harmony (for the system, not you) and not away as I remember. This is all just pure speculation.
Wikipedia - "Sympathetic resonance or sympathetic vibration is a harmonic phenomenon wherein a formerly passive string or vibratory body responds to external vibrations to which it has a harmonic likeness."
Originally Posted by lloydjacobson
After re-reading some of the other posts I'm thinking about tossing the idea from the Chrysler Engineer as exhaust drone. I'm going with 2 ideas... Harmonic balancer or transmission vibration. My reasoning is that the vibration is prominent on the steering wheel and gas pedal. Someone said they changed out the harmonic balancer which resolved their issue. Since the harmonic balancer is made of rubber and metal, temperature can easily affect the characteristics and not eliminate certain vibrations that are naturally caused by the engine. OR the vibration is coming from the torque converter and the rubber isolators and dampers are less effective during cold weather.
If the person who changed out there harmonic balancer reads this please reply or send me a private message.
If anyone has more information about the torque convertor please reply or send me a private message.