Start with wheels and tires, as very often, speed dependent vibrations that you can drive into and out of, turn out to be balance. Spin the wheels slowly and look at the rims for a hop (a flat spot), especially on the inside rim where the wheel is softer and flat spot is harder to see.
You might have just lost a wheel weight too. But, if you see a flat spot, even if the wheel can be balanced, I'd move that wheel to the rear...hoping you're not staggered...though you probably are, right?
Anyway, start there. Tires do also themselves have issues...and if you have a flat spot on wheel, the tire may also have a messed up belt inside...don't really know how you check that other than to remove tire and look. I'd start there before replacing components.
I'm so depressed. No one wants to come see me in NYC: Sat, Oct. 8, 6 p.m. at Greenwich Village Comedy Club.
Mangofy your E46!