I've been assuming I have a loose headset - when I do the headset test (hold front brake and rock bike) I hear a click and feel significant movement beyond just fork flex. But upon closer inspection I found that the front disc brake pads are the source of the movement - when I rock the bike with the front brakes tight I can see the little lever on the brake pads moving 1/8". The rear brake pads don't move at all when I do the same test on the rear.
I'm surprised that pads can come loose, condering that they just slide and click in. I haven't tried reseating or replacing them yet. Don't know if this is related, but the inner pad adjustment knob fell off at some point.
The brakes are Avid Road ball-bearing disc. The wheels are 700c on Hope XC disc hubs.
that the front disc brake pads are the source of the movement - when I rock the bike with the front brakes tight I can see the little lever on the brake pads moving 1/8".
Followup - Avid's customer service is sending replacement "pad retraction clips" and a new inboard adjustment knob, no charge.