Going down a long, fairly steep (well, for Texas - humor us, guys) hill today. I went around a curve and about 2/3 of the way through the curve, the bike starts shaking, rattling, bouncing all around. I was doing about 30. My first thought was, "Oh, man, it's going to hurt when my face hits the pavement going this fast." The second was that the front wheel had come loose and was bouncing around in the frame - the only thing I could think of that'd cause something that bad.

Finally, by the grace of God, got it slowed down without doing an endo. Pulled off the road to see what was wrong, and couldn't find a thing. Wheels were tight in the frame, no visible warping, brakes weren't rubbing - couldn't find a thing wrong. Rode a little ways on the flat side road - fine. Got back on the road, rather gingerly - fine. Did another 35 miles, including some longer, slower hills (never got above 25) - fine.

The pavement seemed fine - I supposed there could have been some grooves I didn't see, but there were no visible problems with it as I was descending. Any ideas?

Lemond Zurich with Rolf Vector Comps and Ultegra, if it matters.