I have a Lionel Atlantic 4-4-2 Milwaukee Road locomotive, paired with a Lionel tender marked “2671W-6” and “THE MILWAUKEE RAILROAD”.  The tender and locomotive each have a single wire tether that connect the two. 
(I have a Lionel type RW transformer (speed/voltage control, Direction and Whistle buttons). 
When the two are tethered, and the locomotive is moving, the tender produces chuffing sounds.
Does anyone recognize the type of tender circuit board?  Should it be capable of producing a whistle sound?
Here’s what I know -
The transformer speed control produces variable AC voltages as expected. 
The Whistle button doesn’t result in a whistle sound; while pressed it increases AC by ~2.0 V, and adds ~0.5 V DC.
I’ve tried adjusting each of the 2 blue variable resisters, but it had no effect on the chuffing sound, and still no whistle.
Any suggestions for getting a working whistle would be appreciated.