I have been lurking and learning, but the search function has not been able to answer my question.

The 4th cog rattles no matter which ring I am on. It makes more noise when under heavy load (up hill), but I hear it under light load too as long as I am pedaling. In my inexperienced opinion, the bike shifts flawlessly and I do not hear any other unusual noises in the other gears. Everything appears to be straight and tight and the only thing that looks unusual is the gap between the 4th and 5th cog may be slightly narrower than the others.

The parts involved:
Hub- Shimano Deore LX
Cassette- SRAM 970 mountain bike 11-34
Chain- SRAM 970
Rear derailleur- Shimano XT
Shifter- Shimano Dura-Ace bar-end

I bought this bike slightly used off of ebay last year. It was custom built as a commuter, but due to a job change it was never ridden. I have put about 1100 miles on it so far, and I am planning on riding RAGBRAI in a couple of weeks. My bicycle mechanic experience is limited to minor bike fit adjustments and cantilver brake set up. I need to learn how to maintain my bike, but I am not afraid to seek professional help if it is over my head. I just don't want to be the guy that hauls his bike to the LBS everytime it makes a funny noise.