I took my bike around the block at about 13MPH and hit every sharp lip that I could find.

There is a rattle that sounds like Wally is describing. If I hit a small, sharp edge at just the right speed it sounds like the Di2 cables are hitting the bottom of the stem or maybe rattling in the internal routing.

I never noticed it before because I am never in the that speed range except when climbing. And I am too busy cursing when I go upslope.

Wally, have you managed to close in on the sound? It doesn't bother me, but maybe you have better ears?