My chain is a SRAM PC-971, worn less than .75", and my derailleur is 9-speed 105. I just installed a new Shimano HG-400 cassette, and it seems like the chain isn't 'meshing' correctly, causing a rhythmic crunching noise as it jumps from the jockey wheels to the cassette (video, worse when under load). This happens in all gears, big/small chainring, with the B-screw in all the way and with it removed completely. The bike shifts well and otherwise performs fine. Is this noise/misalignment anything to worry about? Will it go away over time?
The chain checker tools are only an indicator and in my experiences are relatively coarse.
Your results show that the chain is indeed skipping on a cog. Possible causes:
- Chain is worn or damaged. Grab two adjacent links between forefingers and thumbs, and feel for play. "Barely perceptible" is okay, much more indicates wear.
- Chain is too long and has insufficient tooth engagement. Ensure there's barely enough chain to be in big-big without binding.
- Cassette is damaged or poorly made. Might have burrs on it.
- Indexing is a bit off and the chain is not sitting clearly on one cog. Twiddle a barrel adjuster and see if this helps.
- Chain is not consistent across its length - you many have a stiff link, or maybe a damaged master link.
I had the stiff link once, but only in one direction when going over the top jockey wheel. It was not easily visible even when standing beside the bike. Try and mark the chain with permanent marker and see if the skip happens at or near the same link, or some other commonality.
Good luck ! These problems can be challenging to isolate, but a new chain may be the quickest and easiest solution. If it doesn't fix the problem, then you have a spare chain for the future.