OK! So, my bike is about 2 months old and has around 2,100 miles on it. I used a Park chain checker and it says my chain is in need of replacement. At this point throwing a new chain on will definitely cause some wicked chatter as the chainrings and cassette are showing signs of worn teeth. So should I
A) Suck it up and replace the entire drive train every 2 months or so? EXPENSIVE
B) Replace JUST the chain and the big chainring because that is the one that shows the most wear and I use the most. Allow for chatter when in other gears until the chain wears in on the old gears.
C) Ride it till it doesn't work and then replace it all.

Opinions?

And the amount of cogs in the back = ?
If you're running 7/8 speed, i'd just hit that **** into the ground, cogs = $20, chain = $10, waste more time thinking about it. If you're riding expensive **** then it's your call i'd say. How much is chain replacement worth? Chain AND cogs?

its a 9 speed 11x34 cassette in the rear and a triple tiagra crankset.

Replace the chain. If you find that the new chain slips on the old cassette then replace the cassette as well. It would be quite unusual for a chainring to be worn in only a couple thousand miles - note that the rings frequently have some of the teeth made with a different shape to facilitate better shifting.

Although the rate of wear is very dependent on your riding style and conditions here's what I find to be typical lifetimes for the different components; chains: 4000 miles, cassettes: 15000 miles, chainring: 100,000 miles.

I highly doubt your rings are worn out.

try replacing your chain and see if it skips. if it does, put your old chain back on and ride the drivetrain to the ground then replace the rings, cassette and chain

You seem to understand how a bicycle drivetrain wears but toss in worthless numbers, wtf?

Ok, so I just threw on a new chain a Sram 9 speed PC 991. There is a massive chatter as I ride, It happens in all 3 chainrings and all 9 cogs in the rear. It is very loud, what would this mean?

These numbers don't add up . . . a) 2100 miles in 2 months? You are a better man than me b) What is the chatter? I've previously rode on the same chain for 18 years and it didn't make a sound. Are your FD and RD adjusted properly? Hubs, etc?

My new SRAM chain doesn't make a peep, I'm not sure it is your chain. There is skipping, and then there is noise. I don't see why a properly lubricated and cared for chain shouldn't last at least a full year. Is this a cyclocross bike or something?

Did you ever stop riding to clean and lube the chain? If you plan on ridng that much every month, you should move up to some decent quality parts that don't wear so quickly.

I'd like to see a picture of the worn big ring. It should take at least 10 times that mileage to wear one out. Cheap chains and sprockets could wear out that fast, particularly if ridden in nasty conditions with insufficient maintenance.

A Park chain checker is a poor tool for measuring chain wear. It can report twice the actual elongagation, or more.

I don't know why, but this new chain is RIDICULOUSLY loud non stop. I'm going to put the old chain back on and just deal with it. I clean and lube my entire drivetrain once-twice a week. The original chain is a SRAM 9speed chain and the chainrings are sugino.

Check the routing of the new chain through the rear derailleur and make sure it's going over the pulleys properly and not rubbing against the little tab that's between them. If you turn the pedals by hand while watching the chain you should be able to see where the chatter noise is coming from.

To operator:
The number of miles will vary with conditions but the ratios should still be about the same. I.e. chains generally wear out faster than cassettes and cassettes wear out faster than chainrings.