Laza

Hello all,

I have an old Peugeot City Express (1985 maybe) that I use as a commuter bike. My daily mileage is about 20 miles round trip, I have it for the last 3 years. Most of the bike parts are original except chain which I replaced 4 months ago. When I try adjusting the derailleur chain/rear derailleur sound and shift perfectly. Once I start riding and there is a tension in the chain especially in lower gears (biggest cogs in the back) the derailleur or chain or the cassette (can't figure out which) make crunching metal to metal noise. I understand that the drivetrain is old, but before replacing it all I wanted to check is there any other solution that I am missing. The rear derailleur was never replaced so it's are pretty old (Shimano RD-M531, 3x6), if I have to change the derailleur, which never model will be compatible? Should I change the cassette and chain as well.
Many thanks in advance,
Laza

oldbobcat

I don't know how you adjusted your derailleur to make it work "perfectly," and I don't know how old the rear gear cluster is, and I don't know exactly where the sound is coming from, so I don't know where to start.

OK, I do know where to start. Get the bike on some sort of work stand. Hang it on hooks from the ceiling if you have to. And run through the gears to try to make it make the noise. All noise on a bicycle comes from some sort of movement, so find the movement and components that are making the noise. Then figure out what needs to be adjusted or replaced to make it stop making the noise. If you're puzzled along the way, tell us what you found and ask more directed questions.

HuffyMaster3000

^
Adding to that I would check for wear at the cassette. Make sure the rear derailleur was adjusted in middle gear. Check the cabling.
Also if you didn't notice any noises with the old chain check the new chain for defects.
Inspect the derailleur for damage and if all else fail change it.
There is a possibility there could be something wrong with the rear hub too.

cranky old road 

Is the chain routed through the rear derailleur correctly? There is usually a guide between the jockey wheels that the chain can rattle against if it is not run under it.