Let me just say I totally suck at tinkering with my bike. Now on the eve of a big honking bike ride tomorrow at dawn I'm checking out the shifting (like I could do anything if I found a problem anyway) and I noticed that when I am in my 39X25 ( my smallest gear btw)the chain bumps into the top sprocket of my rear derailer ever so slightly causing a slight knocking sound. Which friggin' screw do I fiddle with?