Lactic acid in my quads seems to keep me company. Or sweat running in to my eyes. Or that little noise from a slight derailer drag on the chain. Or a little "tick" as the plastic decal on my wheels passes lightly by the brake pad on each revolution. Or that slight click I feel in my right pedal as it passes 6 o'clock.

Mostly I try to stay above 15mph because I love the sound my Contis make when going fast.