I first noticed this when moving a short cut of Kadee boxcars at very low speeds - or just on starting [momentum is on]. The cars seemed to jiggle, making small noises as if they were stuttering back and forth. With sufficient drag this wasn’t noticeable, but with 2 or 3 cars the noise was annoying. It looks like the noise is coming from the couplers, but the vibration seems to be coming from the engine. This only happens at low creeping speeds.
Is it a gear alignment artifact? Or something that CV settings can affect?
I’d suggest cleaning the gears and re-lubricating them. Also, watch the valve gear to make sure it’s not binding up anywhere. Put a tiny bit of oil on any valve gear pivots or other moving parts and run the engine around at moderate speed for a while to work it in.
As always, make sure you use appropriate lubricants for model railroad applications, like Labelle oil and grease. These are formulated to be effective and plastic-safe.
This could be caused by the BEMF setting in the decoder. One way to know for sure is to temporarily turn off BEMF and see if the loco performs more smoothly. If not, then BEMF can easily be turned back on.
OTOH, I’ve had similar performance out of several P2K, Spectrum, and other locomotives at low speed.
I had the same problem with a Bachmann Spectrum 2-8-0 Consolidation after installing a TCS decoder and trying it at 28 speed steps. When I tried it at 128 speed steps every thing went smooth. I measured the initial speed at 0.02 mph (106 scale feet per hour). A real pleasure to use.