This is going to be difficult to describe without seeing it first hand, but I'll try.
Recently replaced my 9 speed 12-25 cassette (old HG70, new HG50 aka Tiagra), 42t ring, and chain (old shimano HG73, new SRAM PC-951), just made a small barrel adjustment and was good to go. Test ride was great, drivetrain shifted fine through the whole range and was quiet aside from my derailleur cage rubbing my chain on cross gear settings, but I don't think that's related.
Here's my problem: under torque, specifically standing climbs and/or forceful starts from a stopped position, I get a metallic pinging noise in middle front (42) - large rear (25) combo. I also feel a slight popping in the chain when pedaling. As soon as I shift one cog over (23) the noises disappear. There doesn't appear to be any skipping/slipping as best I can tell. Again, this is only under high torque - this gear combo on flats or in the workstand run fine.
Last night I put the old cassette back on, tried to climb on the 25, and the noise was gone. New cassette, noise is back. I tried loosening/tightening the lockring and backed the RD low stop away from the spokes to no avail - while initially I was afraid the noise might be the chain hitting spokes, I'm leery since swapping cassettes makes it disappear. Visually, my RD cage appeared to be in the same position relative to spokes with either cassette on the 25 cog. I can't imagine there's a width difference between the HG70 and HG50, so RD position shouldn't have to be changed.
I know this is vague without experiencing it first hand, but any ideas? I can't replicate the problem in a workstand, it has to be under torque.
I think your explanation of the problem is clear enough. The solution may not be. The new chain is stiffer than the old one and may not move on and off of that cog as well as the old chain. The teeth on the new 25 cog may not have as smooth an edge as the old cog. It would be interesting (but not practical) to see if a new Shimano chain would run smoother and quieter.
You could try adjusting the "B" screw in to move the rear derailleur upper pulley farther from the big cog, but I doubt that is the problem.
Be sure your chain has enough lube.
It may become quieter with time. Check your new chain often for stretch.
First thing that comes to my mind is your RD cage is rubbing on the spoke crossings. The torque could be pulling the spokes closer to the RD cage (depends on lacing). This does not explain why the old cassette doesn't make the sound but it does explain why you can't duplicate the noise on the stand. A very slight adjustment of the low limit would fix this.
Edit-Sorry should have read more carefully. You checked this out already.
I used a triple crank for several years. If you consider the chainline in the middle ring and largest cog, you realize that is is nearly as extreme as the big ring and largest cog on a double. That is considered to be an off limits combination. I tried to never used the middle ring and largest cog. It's also why I chose a 39T middle ring, so the 39/23 was as low as a 42/25.
Since the problem only occurs under torque, it's even more likely that it's chainline issue. Surely you can tell the difference between the chain and cogs making noise versus something like the RD cage hitting the spokes.
I used a triple crank for several years. If you consider the chainline in the middle ring and largest cog, you realize that is is nearly as extreme as the big ring and largest cog on a double. That is considered to be an off limits combination. I tired to never used the middle ring and largest cog. It's also why I chose a 39T middle ring, so the 39/23 was as low as a 42/25.
Since the problem only occurs under torque, it's even more likely that it's chainline issue. Surealy you can tell the difference between the chain and cogs making noise versus something like the RD cage hitting the spokes.
Thanks for the replies everyone.
I intentionally loosened the L-screw to hear what the RD cage brushing spokes would sound like (never had the pleasure before) and it produced more of a tinny rhythmic/constant noise from contact w/spokes unlike what I'm hearing which is more of a metallic stress/popping noise with random timing. Also tightened the B-screw just to rule that out with no effect, so I'm pretty confident it's not the cage banging into something.
DaveSSS - I think the chainline probably has something to do with it; I can get the pops in my garage easily on 52/25 and 42/25 by slamming on the crank from a stopped position, couldn't replicate on 30/25, but I'll have to take it outside and do a standing hill climb to make sure there's no noise on that combo.
Still perplexes me why my old cassette (HG70 12-25 9 speed) doesn't produce the noise (with the new chain), it's gotta be the same width spec and chainline to the large cog. While 42/25 may be a moderate cross-chain as you said, that's been my favorite standing climb gear for years on shorter high grade climbs and I've never had issues apart from maybe accelerated wear & tear.