Why is 2nd largest rear cog making clicking noise? I have constant clicking noise from the 2nd largest rear cog on my cycle. It starts as soon as I shift into the 2nd gear either from 1st or 3rd gear.

I have checked the following things:


Cable tension - all gear shifts are smooth and quick (both on stand and while riding).
Limit screws are fine.
I have cleaned/oiled the chain and wiped off excess lubricant.
Checked if any of the links are sticking/getting stuck to its adjacent link- they are not.
Visual inspection of teeth shows no excessive wear or bent teeth on any of the cogs.


I have been unsuccessful in trying to locate the problem. I would be grateful for any pointers that will help in troubleshooting this problem.
I am learning to fix my bike and would like to avoid taking it to LBS if possible.

Information: Cogs are 9 speed with Deore derailleur.
It would be one of a few things:


The rear derailleur is not center aligned on the rear cog when in that gear. Take a look at the cog and derailleur gear alignment dead on from the rear when in the noisy gear and look for vertical alignment. If it is misaligned it can be ajusted with the barrel adjuster.
The chain is not clearing the front derailleur guard when in this gear. Look at it from the top down when in the noisy cog and ensure no rubbing between the chain and derailleur guard. If there is rubbing the limit screws can be adjusted
The cog is damaged in some way causing some sort of misalignment


The problem with the first two possibilities is that I'd expect the noise to persist in the 1st cog as well.