It doesn't seems to happen with my foot on or off the pedal, although I may not have paid attention to it sharply enough. And it doesn't seem to happen during turns or at a particular speed. For instance, the first time I noticed it I was cruising down an LA street at probably 30 miles an hour. It just seems to happen but maybe I should pay better attention to the how and when.

You guys might be onto something with the bearings though.