yellow porous rectangle, I noticed the same thing when I built mine. Everything felt smooth, but the belt definately bounced. After a few runs on the track, it's not as bad, but still does it some. I don't know if it's just the angle of the pullys in relation to each other, or the belt itself, or what, but it doesn't seem to hurt anything on the track.
Think you'll be at TCHR on Wednesday? I hope to make it, have to try some stuff out.