How does Mighty Sound Of Steam work?

The smoke unit has an electrical contact on the lever that operates the smoke unit's piston.
Maybe the lever sticks a bit at high speeds.

There are contacts associated with the smoke unit. Depending on the model the contacts may be dirty or they may not be making a complete connection at high speeds in that case you could bend them so they're closer together in the open position. It's also possible the smoke unit isn't properly grounded. 

If your MSOS has a  power roller and grounded wheels on the tender check those connections first. As dirty wheels can also cause intermittent sound.

I found and cleaned the contacts, and it helped a bit. I will try cleaning the wheels on the tender and bending the contacts a little closer tomorrow.