Bicycle Mechanics - Cassette problem?

I recently bought a used Trek 1000 and I think there's a problem with the cassette.

With the front derailleur on the middle ring, the 3rd smallest cog makes a squeak when peddling moderately hard.

With the front derailleur on the big chain ring, the chain skips on the 3rd smallest cog.

There doesn't seem to be a problem with the two smallest cogs. And I hear a metallic jangling sound sometimes from the rear, a bit like give a shake to a couple of coins in the hand.

Are there some things to check? Do I need to get a new cassette? Any chance it could be a problem with the Sora derailleur in the rear?

Thanks for any ideas!

I recently bought a used Trek 1000 and I think there's a problem with the cassette.

With the front derailleur on the middle ring, the 3rd smallest cog makes a squeak when peddling moderately hard.

With the front derailleur on the big chain ring, the chain skips on the 3rd smallest cog.

There doesn't seem to be a problem with the two smallest cogs. And I hear a metallic jangling sound sometimes from the rear, a bit like give a shake to a couple of coins in the hand.

The first thing I would do is check the chain for wear. Once you're certain the chain is good then focus on the cassette.

If you're comfortable doing your own mechanical work then removing, disassembling and reassembling a cassette is easy. You need a chainwhip tool and a lockring tool (http://www.nashbar.com/profile.cfm?sku=9466). With the chainwhip and lockring I'd check to make sure the cassette is tight and snug -- just to make sure the problems aren't caused by a loose lockring.

If the cassette is tight and snug, then i'd disassemble the cassette to see if maybe the 3rd cog is cracked or the spacer is broken or missing.

Correction: Sorry, i was incorrect, the Sora 8-spd cassette has no separate spacers and the 3rd cog is part of the main cassette body.

http://img17.exs.cx/img17/5420/soracass6cj.gif

You might have to replace the whole cassette.

I recently bought a used Trek 1000 and I think there's a problem with the cassette.

With the front derailleur on the middle ring, the 3rd smallest cog makes a squeak when peddling moderately hard.

With the front derailleur on the big chain ring, the chain skips on the 3rd smallest cog.

There doesn't seem to be a problem with the two smallest cogs. And I hear a metallic jangling sound sometimes from the rear, a bit like give a shake to a couple of coins in the hand.

Are there some things to check? Do I need to get a new cassette? Any chance it could be a problem with the Sora derailleur in the rear?

Thanks for any ideas!I'd take it to a bike shop.

the cassette is loose. tighten the lockring

I took the wheel to the LBS today. First, I should mention that the cassette is a SRAM rather than Shimano. Anyhow, the lock ring was a little loose, but so were some setscrews that hold the gears together. Those were tightened up for far less than if I'd bought the tools.

Tomorrow, I'll hope to take a ride to see how the cassette works out.