I had the same issue... When I installed the frame stiffener the steering box was moved slightly and that caused my alignment to be off. I also had the exhaust system rubbing, but not against the frame. It was above the rear axle and up toward the top of the hump in the muffler portion of the exhaust system. It took forever to figure it out and only occurred at high speeds. It sounded like a very low hum, but you could feel it more than you could hear it.
Other cars include: 1976 Lancia Scorpion
2007 BMW Z4 M Coupe