05 Tundra V6 making odd noise between 56-60 MPH. Dealer says cant fix.

I have a 05 Tundra reg.cab that has 2500 miles on it. I really like the truck, gets fantastic gas mileage but since it had about 200 miles has made this very annoying noise when you get to a steady 56 mph. It is a constant exhaust/engine/driveline resignating noise. It is a constant mmmm.mmmmm,mmmmm up and down noise and gets very annoying. If you turn off the OD or drop down to 55 or go above 60 goes away. The problem is that is my main driving speed 80% of the time. The dealer says its a harmonic inbalance and there was nothing they could do. Had 2 Tacoma owners complain also about the same noise. It was a lot worse today when I was towing my trailor with my lawn equipment, was so loud at 59 mph even with window down and radio on still was very noticable. Any help appriciated.