Ever since I replaced my CS-5500 cassette and HG-73 chain with a CS-6500 and KMC X9, my drivetrain hasn't been as quiet. In some gears I can hear occasional clicks coming from the back. Furthermore, if I spin the pedals backwards in the higher (smaller) cogs, I head a "thuk-thuk-thuk" sound every 10 links or so. The shifting is great and I can't feel anything in the pedals, and the sounds don't seem to get worse when sprinting or climbing. No chain skip either. Is the X9 chain just naturally noisy? Does the cassette need to wear in?

Probably a combination of a slightly noisier chain and cassette, combined with he need for some break in.

There's an ongoing trend to make shifting more capable under load. That means more aggressive shift gates (those cutouts on the teeth) and grabbier plates. It's sort of like riding on a train. The tracks a re smooth and quiet but you get a clicking as the wheels pass over the switches.

The occasional click you get when back pedaling is probably due to chain line. When you're on the outer sprockets, the chain is coming from inboard. So there's always a chance of snagging on the tip of a tooth on the next larger sprocket. This is normal and doesn't matter. In any case it usually goes away (or gets less noticeable) as a bit of wear rounds off the edges o everything.

Lastly, and please consider the source here, chain lube can make a big difference. Factories are always dabbling with various lubes, so the new chain might have a different lube than the old one. If it bothers you, and you don't want to wait out more break-in, or it doesn't improve, try using a chain lube with a reputation for making drive trains quieter.

Spinning forward, the jockey wheels keep the chain in line feeding onto the cog. Spinning backwards, the chain can be angled going onto the cog depending on the cog and ring, so it's not unusually to hear it make noise. Watch it will spinning the crank backwards while in a small cog and on the small ring and you'll see the chain catching on the adjacent larger cog.

Occasional clicks pedaling forward would suggest the cable needs adjustment. Most often it's too tight and the chain is trying to climb to the next larger cog. This is easy to see spinning the crank with the rear wheel supported off the ground. Viewing from the back, the chain should be feeding on inline with its cog and not rubbing on the next larger one.

Clicks under hard pedaling might be due to a loose cassette. I've had mine get loose after a while and need to be retorqued.

At the risk of others' flames I'll say that i don't find the KMC chains to be up to the level as Shimano and SRAM. I know that KMC is Shimano's contractor, I've had this discussion with a KMC guy at Interbike already. But in the last few shops I've worked in (since 1990 approx) we've found the KMC chains to tend to not shift as nicely and wear faster. Since about back then we haven't consistently sold the multy gear KMC chains over the counter but see enough on new bikes and repairs to feel that we have had a good population to judge from.

The OP's mentioning of the smaller cogs having an approx 10 link clunk/click would match the single shift gate on these small cogs. I find that MANY bikes have the hi gear (smallest cog) limit a bit too tight. The chain will drop onto it well enough but any cable tension increase (say when really hammering) will cause the chain to slightly snag on the next cog's shift point. Andy.

+1 on the KMC chains Andy, I fear this will date me, however I recall Renold, Sedis Gold, Sachs, and SRAM being great shifting and long wearing drive chains compared to KMC. IMHO Shimano chains all shift great, are smooth running, but the wear characteristics are slightly less than the others.
Carl

Did you readjust your rear derailleur when you changed out your cassette? Even changing like for like may necessitate minor adjustments due to manufacturing tolerances.

+1 for SRAM chains.

The interesting thing about anecdotal chain life data is that it varies so much. As an old timer I can attest that Sedis (made in France), and D.I.D. chais had great wear life, but for Sedis, the quality dropped noticeably when they moved to Portugal.

In any case, you can't compare old chains to new chains because newer chains are narrower and limited in then hardness allowable for the plates.

But when we compare modern chains, we find that for just about everyone that says a particular chain is better, there's another who says it's the worst he's ever ridden. IME, all the chains in the upper 1/2 to 1/3rd of the various companies lines are within a very narrow band in terms of wear life. I find the best $$$/mile value at the best chain below those with better plating, or punched links.