Putting together a bunch of leftover parts onto an old MTB frame.
I have a NOS Suntour 12 tooth - 1/8 cog. For a chain I have a cheapo Bell BMX (but I suspect it is a rebranded KMC).
The chain is VERY noisy, and I can see the chain fighting to sit down in the gaps on the cog as it turns. I can easily see the chain throbbing up and down when spun quickly.
Chain tension seems not to be the problem; the chatter is there at high and low tension.
Chainline does not seem to be the problem. I have it dialed in perfectly. In any case, I tinkered a bit to move it inboard and outboard a smidge, and there was no change.