It sure sounds to me like the tire picked up something in the road right after you rotated the tires,,coincedental?
Have you had the tire inspected?
The only other thing it could be is there might be a small bubble in the tire itself and wont be able to hear it at lower speeds, pretty common.
Other than that i cant think of ANYTHING else that would cause this after rotating the tires...
If you have any further questions, please feel free to reply.
Thanks for asking!