The other day, after leaving from a redlight, I was shifting through the gears. while in second the tach hit 3 grand, after I was sunddenly distracted. No problem, I think, this has happened before. At the next light, I noticed a high frequency vibration in the steering wheel and shifter, that wasn't there before. Very subtle, but I can still notice it. It doesn't matter if the clutch is in or out at idle, AND the vibration goes away with any increase in rpm's. Before all this happened, the valves had been recently adjusted, and the fuel filter is brand new. Not to throw another wrench in the machine, but I had just filled up 8 miles before with another brand of diesel I had never used before. The truck has not lost any power and is not any louder. any ideas?