Joe,
The fact that there was no significant accompanying vibration is pretty reassuring. I think you probably solved the problem... some sort of wind induced harmonic effect on the bottle cage sounds likely. You guys in NC are the exception, but most of us spend very little time at really high speeds on our bikes. I learned last summer riding in Montana that going down a mountain is a whole different world where a wind gust or mechanical issue or lapse of attention (or loose article of clothing) can be extremely scary. Experience with this is important and it is wise to be cautious if you value life and limb.

Be careful out there and I look forward to hearing about your RtR experience.