What brand/model tires did you get? I bought Falken FK 452's and had a "...thrum...thrum...thrum..." that was dependent on road surface type. It would do this on tarmac, but not on concrete. Weird. I was told by a Honda S2000 owner that his did it and the reason given to him by Honda was "a directional tread pattern sometimes does this on short wheelbase cars."

I have not heard of this from owners that keep true to the OEM Continental's the car came with-only directional tread tires.

Hope this might be an explanation.