haha, I had that one just the other day!

Also on that ride, at one point I found a water bottle on the side of the road (my favorite kind, Camelbak Podium), so I secured one of my in-use water bottles on my rack with a bungee net. Due to careless securing, a little bit of bungee was lightly scraping the top of my rear tire under the rack. Just lightly enough that it stopped and started cyclically every revolution. Fortunately I was able to eliminate brake rub and tire-on-frame rub pretty quickly, and a closer inspection found the offending bit of bungee