Sounds like you have covered most of the bases ie. chain, cassette and derailleur. Was the new chain sized correctly? From your description of the problem it is difficult to diagnose, is it actually making a noise when riding in that cog or is it just a rumbling through the pedals that you are feeling/hearing? With regard to going to a single up front your current middle chainring should be your ideal chainlne so a single should make no difference. Things to consider...how worn are the chainrings, if the noise/rumble? feels like its coming from the rear is it time to overhaul the hub +freehub? then there is the front end..bb + chainline to consider since it only happens under load flex at the bb end and loading on the hub/freehub may provide an answer..good luck