Chain noise with a new cog
 

My chain has started to make a lot of noise ever since I replaced my cog and I've been trying to figure out why. I don't want to waste money on parts I don't need, so I hope you guys can help me narrow it down. Here's the facts:

I was riding with an EAI deluxe 15t cog (the black one) with an Izumi Eco chain. My bike ran pretty quiet. I switched the cog with a new EAI superstar 15t cog (with an orange pouch) and after a couple of days, my bike starts making a relatively loud, annoying noise.

I cleaned and relubed the chain and the sound went away for a couple of days then came back. I've made sure to check that the chainline is straight. Some people have suggested that my chain has stretched. This could be, since I've been riding it about 10 miles a day for ~2 years now. But it still doesn't explain why, with the old cog, there was much less noise. And unfortunately I got rid of my old cog, so I can't switch it back in.

Does anyone have any recommendations about what to look for? I plan on going to the bike shop this weekend to check if the chain has, indeed, stretched. But if there's anything else I should check, I'd like to know.

Thanks!

Im thinking not only your chain has streached but your chainring has also changed along with it.

Both your old cog and chain wore together, so they remained quiet. Now that you have a new unworn cog with an old worn (stretched) chain, they don't mate smoothly and so you have noise and roughness. The solution is to buy a new chain.

This makes sense. Thanks!

Can you tell me how to check if my chainring has worn/changed?

Thanks

Compare it to a new one you should see a differnce.

Chainrings last quite a long time, I wouldn't worry about it. Read the following, especially the sections on chain "stretch" and chain and sprocket wear: http://www.sheldonbrown.com/chains.html

Replace your chain so you don't ruin your fancy new cog.

but if the chainring is worn a new chain isn't going to solve the problem

I doubt it is. As Jake said, chainrings don't see as much wear as a cog and last considerably longer.