Do the rollers have signs of wear and pitting discoloration? Cleaning rollers also requires internal. Sometimes they just need to be replaced despite not physically grooved.
I would also open the tender and make sure the plugs fully seated especially the 5 pin motor plug.
The other issue can be the speaker going bad. Turn volume down and retest. If train runs fine with low volume or off (speaker not vibrating), I would try changing the speaker.
The issue maybe that the sound generation (a high demand on the 5V power supply that runs all the microchips) is effecting how the motor fet controls speed or the tach reads speed.
G
