I recently did the same thing (old cog w/new chain) and the drive train got super noisy. The harder I pedaled the louder it got. I was surprised how loud it was. I switched back to the old chain and quiet as a mouse. I have a dingle cog, so I just switched to my other set up (50/18 vs. 43/16) and rode a little faster. I will swap out the sprocket ($45) when I switch back to my winter gearing.