Ok, its apparently not speed specific, I must have only really noticed it at 55 when I wasn't distracted by all the other noises.
It is there are lower speeds, resonating at the same frequency. Its actually a bit "stronger" at lower speeds. Now I think it actually comes from the back, and my best guess would be out of balance wheel.
The other possiblity, is that its the engine mounts. I know they are gone; just havent gotten to them yet. Could this be doing it?

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Paul
2004 E500 4matic; 72,000mi