If the chain isn't too old...you have measured it and it is not too long...it's not likely to be a problem with the chain per se. Chains stretch is actually chain wear. If the chain isn't worn down to the point of stretching it's hard to imagine it's a chain problem. Buying a new chain would be informative if the new chain still seemed noisy...then you would know it's not a chain problem...but if it were silent I'd still be puzzled why the other chain got noisy.
One possibility is you aren't getting the simple green out of the chain and it is diluting the oil. It's also possible you aren't getting enough oil into the rollers. Test this by oiling the hell out of it and then cleaning the outside of the chain well. If all that fails then I guess a new chain would be of interest. If it seems to stay quiet for even 10 miles then starts to get noisy I think it has to be something in the cleaning or oiling situation. 2,000 miles is a lot for a 10 speed chain. If it's kept really clean it's not crazy...but most folks will need to change chains before that. Pushing the life of a chain can be pennywise and pound foolish because the longer you try to keep a chain the more the chainwheels and cogs will be worn down. A new chain every 1,000 miles or so will go a long way to keeping rings and cogs from getting worn but it does have a lot to do with how clean you keep your chain. After re-reading your post I would question the idea of simply spraying simple green on the chain. The important part is inside the rollers. Spraying SG on the chain won't get into the rollers. After removing the chain put it in a bath of simple green and shake the heck out of it. Put it in another bath (don't use more than needed) of fresh SG and do it again. Keep replacing SG until shaking the chain and moving it around in the SG doesn't result in the SG looking dirty. Scrub and brush if necessary during the soaking in SG. THEN you know the chain is clean. Rinse the chain in hot water and dry in an oven at 200F. That will result in a very clean chain. Oil very well, wipe very well ( run a bit of cloth inside the gaps to even clean there because any coating of oil on the outside will hold and distribute dust and dirt). If that fails, a new chain is certainly in order.