Ok the enginerd got me, busted out the spectrum analyzer and found my issue freq. I did three tests to isolate the offending frequency and found about a 10db increase at 100 hz.
Here is the results in 6th gear going 60mph at 1500 rpms where the drone is the worst. Look at the yellow line and how it makes a step change after 120 hz.
Now here is the same instance but in 5th gear at 60 mph and 2000 rpms. Notice the hole between the peak 80-120 hz bar it's almost 10db full difference. If you understand sound 10db is a very significant jump.
Just for fun I hit the interstate at 70 mph in 6th around 1800 rpms. The drone is not nearly as bad as 60 mph the graph shows it basically splits the difference.
Now if you reference the formulas and charts floating around on the hemoltz resonators you can see I need like a 33" J pipe to effectively cancel out 100hz.
Not sure how I would fit that in there but it's cool to have a possible answer.
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