I'm getting a clunking (as opposed to clicking) sound that sounds like is is coming from the bottom bracket/rear of the bike when i pedal hard (uphill usually). the odd thing is that i only get this sound when i'm in the second to largest cog in the back. When i shift up or down one, i cannot recreate the sound no matter how hard i stamp the pedals. Any ideas?

Thanks

check that particular ring for any chipped teeth, or shark toothed teeth (pointy), that would be where I would start.