i had the same thing happen. so i figured out that it was the harmonics from the muffler there's nothing you can do except try to remove/replace the muffler. mine actually got better after i hit the snow bank last summer, bending/denting the muffler. but we had another x-fire on the used car lot, so i acquired a new muffler, and low and behold the noise re-appeared.

so i found another sure fire solution, I GOT RID OF THE CAR. Traded it in for the Trail Blazer SS. no more noise...